BEIRUT (Reuters) - Syrian forces bombarded Homs on Monday, killing 50 people in a sustained assault on several districts of the city which has become a centre of armed opposition to President Bashar al-Assad, the Syrian National Council opposition group said. 
 
 Western countries seeking Assad's downfall scrambled to find a new diplomatic strategy after the defeat of a U.N. Security Council resolution backing an Arab League call for Assad to give up power and start a political transition. 
 
 The United States shut its embassy in Damascus and said all staff had left the country due to worsening security. Britain said it withdrew its ambassador from Syria, and would seek further European Union sanctions against Syria. 
 
 Russia fought back against blistering criticism from the West for vetoing the resolution on Saturday. Foreign Minister Sergei Lavrov, who is due in Damascus on Tuesday, said condemnations of Moscow's veto had verged on "hysteria." 
 
 U.S. President Barack Obama said that, however hard Western countries are prepared to lean on Assad diplomatically, they still had no intention of using force to topple him, as they did against Muammar Gaddafi in Libya last year. 
 
 "I think it is very important for us to try to resolve this without recourse to outside military intervention. And I think that's possible," he told NBC's Today show. 
 
 The opposition Syrian National Council's (SNC) Catherine al-Talli told Reuters shelling of Homs early on Monday killed 50. 
 
 Assad's opponents say his tanks and artillery killed more than 200 people in the city on Friday night in the bloodiest incident of the 11-month-old uprising against his rule. 
 
 That attack, branded a "massacre" by France and "unspeakable" by Obama, set the stage for intense efforts over the weekend to lobby Moscow not to block the U.N. Security Council resolution. 
 
 But Russia argued the resolution was one-sided and would have amounted to taking the side of Assad's opponents in a civil war. China also vetoed the measure, by most accounts following Russia's lead. 
 
 "It is sad that the co-authors decided to hastily put the resolution to a vote, even though we appealed to them with a request to give it a few more days" until after his own planned trip to Damascus, Lavrov said. 
 
 "Some of the voices heard in the West with evaluations of the results of the vote in the U.N. Security Council on the Syria resolution sound, I would say, improper, somewhere on the verge of hysteria," Lavrov told reporters after meeting the foreign minister of Bahrain, one of the Arab states that has sought a tougher stance against Assad. 
 
 RUSSIAN ROLE 
 
 Lavrov has said Russia favors a peace dialogue in Syria that is free of outside interference and preconditions. 
 
 He repeated the message in a phone conversation with Arab League chief Nabil Elaraby, the Russian Foreign Ministry said. Elaraby, asked in an interview with Reuters whether Russian mediation could end the crisis, said: "They believe so." 
 
 The collapse of an Arab League monitoring mission in Syria as violence surged set the stage for the U.N. standoff. 
 
 The opposition Syrian Observatory for Human Rights said it had the names of 43 people killed in Monday's bombardment of Homs. Television footage showed smoke rising from buildings, with explosions echoing in the background. 
 
 "This is the most violent bombardment in recent days," said one activist in Syria who was in touch with Homs residents. Another activist said government troops were using multiple rocket launchers in the attack. 
 
 Damascus denies firing on houses and says images of dead bodies on the Internet have been staged. State media said on Monday "armed terrorist groups" were firing mortars in the city, setting fire to tires and blowing up empty buildings to give the impression that Homs was under fire from Assad's forces. 
 
 State television quoted an interior ministry official as saying six security personnel died in fighting that killed tens of "terrorists," and showed brief interviews with Syrians urging the government to strike with a "fist of iron" in Homs. 
 
 State news agency SANA described attacks in the city by "terrorists" who it said killed a textile factory worker. It said they also killed three officers and abducted several soldiers in Jabal al-Zawiya in the northern Idlib province. 
 
 Reports from activists and authorities are hard to verify because Syria restricts access for independent media. 
 
 IRON FIST 
 
 Activists said there had been explosions in the Khalidiya, Baba Amro, Bayada and Bab Dreib neighborhoods of Homs. 
 
 "They want to drive the Free Syrian Army out," said Baba Amro resident Hussein Nader by telephone, referring to the force of army deserters and rebels who have held parts of Homs for months. "Rockets are falling seconds apart on the same target." 
 
 Another resident, Omar Shakir, said activists had information the shelling would continue until Thursday, when troops were expected to move into Homs. "We have no one but God - everybody abandoned us," he said. 
 
 Activists said a blast hit an oil pipeline feeding a main refinery in Homs, the second attack in a week. Separately, three people died when the opposition-held town of Zabadani, near the Lebanese border, came under fire on Monday. 
 
 Syrian army defectors announced they were organizing a new "Higher Revolutionary Council" to supersede the Free Syrian Army (FSA) as the main armed force battling Assad's rule. The new body would be commanded by General Ahmed al-Sheikh, the highest-ranking officer to defect to Turkey from government forces. 
 
 Some observers said the latest offensive by Assad's forces may have been planned for some time, noting a speech last month when he vowed to strike "terrorists" with an iron fist. 
 
 "The regime 10 days ago took a decision to confront the opposition in a different manner," said Ayham Kamel, of the Eurasia Group risk consultancy. "The tactics before were targeted. Now they are set on more direct confrontation with the FSA and opposition activists." 
 
 "KICK IN THE ASS" 
 
 The Russian and Chinese veto of the U.N. resolution was condemned in unusually harsh language by Europe and the United States, who said Moscow and Beijing would bear responsibility for future bloodshed. 
 
 U.S. Secretary of State Hillary Clinton called the veto a "travesty" while French Defence Minister Gerard Longuet on Monday said: "There are political cultures which deserve a kick in the ass ... To accept that a dictator can operate freely is disgraceful for governments that accept it." 
 
 The Syrian National Council said the Russian and Chinese vetoes of the resolution had given Assad a "license to kill." 
 
 Syria is a long-standing ally of Moscow, one of the few in the region, and a buyer of its arms exports. Russia clearly still hopes to play an important diplomatic role with Assad, and says it wants to encourage him to adopt reforms. 
 
 Fawaz Tello, a senior member of the opposition SNC called the Russian position "shameful" and Moscow's talk of reform hypocritical, telling Reuters: "The Russians know that such a thuggish secret police regime cannot be reformed." 
 
 The veto bore the stamp of Prime Minister Vladimir Putin, who intends to return in a March election to the presidency he held from 2000-2008, when he was known for firmly opposing what he saw as efforts by the West to extend its influence at Russia's expense. 
 
 China's state-run media said Western intervention in Libya, Afghanistan and Iraq showed the error of forced regime change. 
 
 "Currently, the situation in Syria is extremely complex. Simplistically supporting one side and suppressing the other might seem a helpful way of turning things around, but in fact it would be sowing fresh seeds of disaster," the People's Daily said. 
 
 Clinton said on Sunday the United States would work with other nations to try to tighten sanctions against Assad's government and deny it arms in the absence of a U.N. resolution. 
 
 That will be hard without a Security Council resolution and Syria is far less dependent on oil exports than other states in the region, making it less vulnerable to an embargo. 
 
 (Additional reporting by Khaled Yacoub Oweis in Amman, Joseph Logan and Mariam Karouny in Beirut, Nastassia Astrasheuskaya and Alissa de Carbonnel in Moscow, Chris Buckley in Beijing, Arshad Mohammad in Sofia, Annika Breidhardt in Berlin, Mirna Sleiman in Dubai and Leigh Thomas in Paris; Editing by Jon Boyle) ||||| Media playback is unsupported on your device Media caption Paul Wood in Homs says there is "nowhere to hide" for the city's people 
 
 Heavy artillery fire has been rocking Homs, as Syrian troops step up an assault on the restive city. 
 
 A BBC correspondent there describes almost constant blasts, in the fiercest attack in the 11-month uprising. 
 
 US President Barack Obama said it was important to resolve the conflict without outside military intervention. 
 
 Meanwhile, Russia and China defended their veto of a UN draft resolution criticising Syria - a move that angered opponents of President Bashar al-Assad. 
 
 Later the US State Department said it had closed its embassy in Damascus and pulled out all remaining staff because of security concerns. 
 
 Washington had warned in January that it would close the embassy if the government did not step up security. 
 
 'Not safe at all' 
 
 Homs, one of the main centres of resistance to Mr Assad's rule, has been under attack from government forces for several days. 
 
 Shelling resumed shortly after daybreak on Monday, says BBC's Paul Wood who has managed to get into the city, and hundreds of shells and mortars have been fired throughout the day. 
 
 Analysis Syrian state television denied that there had been any bombardment. It said residents were setting fire to piles of rubbish on the roofs of their homes to trick the world into thinking that there was an attack. There is no doubt, however, from what we have seen and heard, that hundreds of shells and mortars have been fired at this place during the day. As I write this, the windows of the house we are in are still reverberating from the impact of a shell, probably in the next street. It is true that people have been setting fire to rubbish in the streets. They believe it will confuse the guidance systems of rockets apparently being fired at them. They are probably mistaken. People in this part of Homs say these attacks are the worst they have known since the beginning of the uprising, almost a year ago. The bombing has been going on for several days now. 'They are still picking up the bodies' Homs: 'Who will help us now?' Avoiding Syria's secret police 
 
 Eyewitness Danny Abdul Dayem told the BBC the army was using rockets for the first time, with more than 300 falling on his locality since dawn. 
 
 "It's not safe at all, a rocket could land in this house right now," he said 
 
 Some rebels fighters have been firing automatic weapons in return, in what our correspondent calls a futile gesture. 
 
 The rebels claim that the shelling has hit a field hospital in the Baba Amr district, causing casualties. However, our correspondent says this is impossible to verify. 
 
 The facility is treating dozens of people wounded in previous assaults on Homs. 
 
 Mr Dayem said only one field hospital with four doctors was still operating in the city, and it was virtually impossible to get additional medication without being shot. 
 
 Another anti-government campaigner told the BBC the government was also using helicopters and tanks in the assault. 
 
 Activists say at least 40 people were killed on Monday. 
 
 Syrian state TV said "terrorist gangs" had blown up buildings in Homs. 
 
 The state-run Sana news agency reported that an oil pipeline near the city had been hit by an explosion on Monday. It also blamed "terrorists". 
 
 Both Syrian media and activists are also reporting clashes in the northern city of Idlib and the town of Zabadani, north-west of Damascus. 
 
 The government says it is fighting foreign-backed armed groups. Thousands of former army soldiers have defected to the rebel side, forming the Free Syrian Army. 
 
 The BBC's Jim Muir in neighbouring Lebanon says the conflict is beginning to look increasingly like a civil war with dangerous sectarian overtones. 
 
 The uprising is largely rooted in poorer sections of the Sunni community, our correspondent says, while the government draws its support mostly from Alawites, Christians and other minorities fearful of an Islamist takeover. 
 
 The Assad regime is feeling the noose tightening around them US President Barack Obama Russia keeps Syria options open 
 
 'Inexcusable' 
 
 The Syrian opposition says Saturday's veto by China and Russia of a UN draft resolution condemning the crackdown will encourage the government to act without restraint. 
 
 President Obama vowed to apply sanctions and put pressure on Mr Assad. 
 
 "I think it is very important for us to try to resolve this without recourse to outside military intervention. And I think that's possible," he said in an interview for the NBC network's Today programme broadcast on Monday. 
 
 He added that a negotiated solution was possible and said the US was "relentless" in demanding that Mr Assad leave power. 
 
 "The Assad regime is feeling the noose tightening around them," he said. "We're going to just continue to put more and more pressure until hopefully we see a transition." 
 
 UK Foreign Secretary William Hague described the Russian and Chinese vetoes as "a grave error of judgement". 
 
 Media playback is unsupported on your device Media caption Danny Abdul Dayem in Homs: "Pieces of bodies, children, women" 
 
 French President Nicolas Sarkozy said he would discuss the situation in Syria with his Russian counterpart Dmitry Medvedev later on Monday. 
 
 "France and Germany will not abandon the Syrian people," Mr Sarkozy said after a meeting in Paris with German Chancellor Angela Merkel. "We will not accept that the international community remains blocked." 
 
 However, Russian Foreign Minister Sergei Lavrov said the resolution, drafted by Arab and European countries, would have meant taking sides in a civil war. 
 
 Speaking in Bahrain on Monday, he said encouraging "armed extremist groups" would only cause more casualties, and said Moscow supported peaceful dialogue in Syria. 
 
 Mr Lavrov is due to travel to Damascus on Tuesday for talks with President Assad. 
 
 Media playback is unsupported on your device Media caption Homs resident: "I watched a three-year-old girl dying" 
 
 The Chinese government also defended its veto. It said the draft resolution would only have complicated matters, and said Beijing sought to "avoid the scourge of armed conflict". 
 
 Human rights groups and activists say more than 7,000 people have been killed by Syrian security forces since the uprising began last March. 
 
 The UN stopped estimating the death toll in Syria after it passed 5,400 in January, saying it was too difficult to confirm. 
 
 President Assad's government says at least 2,000 members of the security forces have been killed fighting "armed gangs and terrorists".
– Syria's regime is raining devastation down on Homs for a fourth day today, with the Telegraph reporting that hundreds of shells are hitting the city with each passing hour. Rebels there have been rendered defensively helpless, incapable of responding with anything more than gunshots. "We can't count all the bodies from the streets and the collapsed buildings," says an insurgent leader. "We are living in a tragedy. People can't even run away or flee, and are dying in their homes." The bombardment continues even as Russian Foreign Minister Sergei Lavrov arrived today in Damascus for talks with Bashar al-Assad, and a day after the US shuttered its embassy. Russia voted against a UN resolution urging Assad to step aside on Saturday, and Lavrov was welcomed by enthusiastic crowds, but the country is said to be pushing its own plan to end the bloodshed, notes the New York Times. “It is in our interests for Arab peoples to live in peace and agreement,” Lavrov said. Others are losing patience: France and Italy today recalled their ambassadors, while British Foreign Minister William Hague yesterday termed Assad's rule a "doomed regime, as well as a murdering regime." Click through the gallery for more scenes from Syria.