Some lawmakers have warned that the operation might turn out to be a largely symbolic strike that would leave the Assad government with the capability to mount sustained attacks against civilians with artillery, rockets, aircraft and conventional arms and would do little to reduce the violence in Syria, limit the flow of refugees or encourage Mr. Assad to negotiate seriously if a Geneva peace conference is convened. 
 
 Representative Eliot L. Engel of New York, the ranking Democrat on the House Committee on Foreign Affairs, suggested in an interview that the attacks go further than what appears to be under consideration by the administration, including strikes on the Syrian Air Force, its munitions depots and military fuel supplies to “tip the battle in favor of the insurgents.” 
 
 “We should try to help the rebels and help the people fighting Assad,” Mr. Engel said. 
 
 Frederic C. Hof, a senior fellow at the Atlantic Council who previously worked on Middle East issues for the State and Defense Departments, has urged that the Obama administration consider a broader military mission: destroying or significantly degrading the ability of the Assad government to carry out intensive artillery, aircraft and rocket attacks with conventional as well as chemical warheads on the civilian population. 
 
 “Something that is significantly less than that, something that is seen as symbolic, I think would just enable Bashar al-Assad to say I have stood up to the world’s only superpower and faced it down,” he said. 
 
 The main American attack is expected to be carried out by cruise missiles from some or all of the four Arleigh Burke-class destroyers within striking range of Syria in the Mediterranean: the Mahan, the Barry, the Gravely and the Ramage. 
 
 Each ship carries about two dozen Tomahawk cruise missiles, a low-flying, highly accurate weapon that can be launched from safe distances of up to about 1,000 miles. Tomahawks were used to open the conflicts in Afghanistan in 2001, in Iraq in 2003 and in Libya in 2011. Attack submarines also carry Tomahawks and are assumed to be on station in the Mediterranean as well. 
 
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 Officials said that while Syrian rocket and artillery sites were expected to be targeted, there were no current plans to use Tomahawks to crater airfields used by the government to receive weapons and military supplies from Iran, an important lifeline for the Assad government. 
 
 Weapons experts said that Tomahawk missile strikes, while politically and psychologically significant, could have a limited tactical effect. The weapons are largely fuel and guidance systems and carry relatively small high-explosive warheads. One conventional version contains about 260 pounds of explosives and another version carries about 370 pounds. Each is less than the explosive power of a single 1,000-pound air-dropped bomb. 
 
 The weapons are not often effective against mobile targets, like missile launchers, and cannot be used to attack underground bunkers. Naval officers and attack planners concede that the elevation of the missile cannot entirely be controlled and that there is a risk of civilian casualties when they fly slightly high. 
 
 Some officials have also cautioned that Iranian-backed Hezbollah militants might step up terrorism around the region in reaction to American strikes on Syria. Another risk is that Mr. Assad might respond to the attack by firing missiles at Turkey or Jordan or mounting even more intensive attacks against civilians. 
 
 Although some experts believe that the Syrian government already has its hands full trying to contain the rebels and would not relish a war with the United States, they say that the Obama administration needs to be prepared for another round of airstrikes should Mr. Assad raise the stakes. 
 
 In an indicator of the complexities within Syria’s civil war, and the difficulties faced by the Obama administration in any effort to guide the conflict’s path, jihadi fighters opposed to Mr. Assad were warning one another to take steps to avoid being hit in any impending American attacks. 
 
 On Monday night, one prominent member of the Nusra Front, a rebel group aligned with Al Qaeda and designated a terrorist organization by the United Nations and the United States, used a Facebook posting to urge fellow members to move away from their bases or positions in Syria. 
 
 “All fighters in Jabhat al-Nusra,” he wrote, using the organization’s Arabic name, “please constantly change your positions and don’t share anything online. There is a conspiracy by America and its tails to hit our positions.” 
 
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 Attacking chemical weapons storage sites comes with the same difficulties and risks associated with attacking munitions depots generally, and with its own special dangers, which the American military encountered in two wars in Iraq. First among them are risks of contamination to the very Syrian civilians that any military action would officially be intended to protect. 
 
 Many veterans suspect that some of the effects of Gulf War syndrome that afflicted veterans of the Persian Gulf war of 1991 were caused by exposure to chemical weapons released in clouds by conventional airstrikes against Iraq’s chemical weapons sites in southern Iraq. 
 
 After the first gulf war, an American Army unit near Kuwait breached chemical weapons while destroying conventional munitions at Khamisiyah, creating an environmental hazard that persisted throughout the American occupation of Iraq after the invasion in 2003. 
 
 Similarly, airstrikes in 1991 on bunkers at the Muthanna chemical weapons complex near Samarra, Iraq, led to security and environmental problems that continue to the present day. 
 
 During the Clinton administration, the United States and its NATO allies carried out extensive airstrikes against Serb forces in Bosnia, which weakened them to the point that a peace settlement to end the Bosnia war was negotiated in 1995 at an American Air Force base in Dayton, Ohio. 
 
 Similarly in Kosovo in 1999, an intensive NATO air campaign that lasted 78 days led to an agreement in which Yugoslav forces withdrew from Kosovo, and the region achieved autonomy and eventually independence. 
 
 Mr. Obama’s national security aides have been studying the NATO air war in Kosovo as a possible blueprint for acting militarily in Syria without a mandate from the United Nations. ||||| Media playback is unsupported on your device Media caption President Barack Obama: ''I have no interest in any kind of open-ended conflict in Syria'' 
 
 US President Barack Obama has said he has not yet decided on a plan for retaliatory action against Syria. 
 
 But he said he had concluded Syrian government forces were behind a recent chemical weapons attack near Damascus. 
 
 Speaking on US television, Mr Obama said the use of chemical weapons affected US national interests and that sending a "shot across the bows" could have a positive impact on Syria's war. 
 
 His comments follow a day of behind-the-scenes wrangling at the UN. 
 
 The UK had been pushing for permanent members of the UN Security Council to adopt a resolution which would have authorised measures to protect civilians in Syria. 
 
 But Syrian ally Russia refused to agree to the resolution and the meeting produced no end to the diplomatic stalemate which has long characterised the UN position on Syria. 
 
 The US State Department criticised "Russian intransigence" and said it could not allow diplomatic paralysis to serve as a shield for the Syrian leadership. 
 
 Russia is sending an anti-submarine ship and a missile cruiser to the eastern Mediterranean. 
 
 The ships are being sent to strengthen the navy's presence in the area because of the "well-known situation" there, the Russian news agency Interfax has said. 
 
 But another news agency, RIA Novosti, quotes a senior naval command spokesman as saying that this is just a planned rotation, unconnected with Syria. 
 
 'Strong signal' 
 
 Critics have questioned what purpose a limited strike on Syria could serve, but Mr Obama told the Public Broadcasting Service (PBS) it would send the government of Bashar al-Assad "a pretty strong signal that it better not [use chemical weapons] again". 
 
 The US has yet to produce the intelligence it says shows Mr Assad's government is guilty of using chemical weapons, and UN weapons inspectors are still investigating inside Syria. 
 
 The team has just begun a third day of on-site investigations, and UN Secretary General Ban Ki-moon has appealed for it to be "given time to do its job". He said the inspectors would finish their investigations and be out of the area by Saturday morning. 
 
 Syria denies using chemical weapons and blames opposition fighters for the attack near Damascus on 21 August, which reportedly killed hundreds of people. 
 
 If there was action it would be to send the Syrian regime a 'pretty strong signal' 
 
 It accused the West of "inventing" excuses to launch a strike. 
 
 In a sign of growing fears about an impending attack among Syrians, the Associated Press quoted Lebanese officials as saying at least 6,000 Syrians crossed into Lebanon in a 24-hour period through the main Masnaa crossing - compared to a normal daily tally of between 500 and 1,000 refugees. 
 
 "Isn't it enough, all the violence and fighting that we already have in the country, now America wants to bomb us, too?" one 45-year-old woman, entering Lebanon with her five children, told AP. 
 
 In Damascus senior military commanders are reportedly staying away from buildings thought likely to be targeted. You "could hear a pin drop" at one of them, a local resident said. 
 
 'Consequences' 
 
 President Obama told PBS that the US had "not yet made a decision, but the international norm against the use of chemical weapons needs to be kept in place, and hardly anyone disputes that chemical weapons were used in a large scale in Syria against civilian populations". 
 
 Military intervention is morally justified Anthony Luder, Safed, Israel 
 
 Intervening now in Syria would be a terrible mistake Demetrios Nicolaides, Limassol, Cyprus Global readers' voices on Syria 
 
 "We've looked at all the evidence, and we don't believe the opposition possessed chemical weapons of that sort," he said. 
 
 He added he had concluded that the Syrian government carried out the chemical weapons attack. 
 
 "There need to be international consequences, so we are consulting with our allies," he said. 
 
 There was "a prospect that chemical weapons could be directed at us - and we want to make sure that doesn't happen". 
 
 The BBC's David Willis in Washington says Mr Obama looked cautious and spoke in a measured way, and he was clearly concerned about getting Congress on board as well as the American public. 
 
 Opinion polls until now have shown very little interest among the US public in getting involved in the Syrian conflict. 
 
 In an open letter to the president, US House of Representatives Speaker John Boehner demanded he explain "the intended effect of military strikes", and how he would prevent the intervention escalating, if he wanted to win public and congressional backing for action. 
 
 More than 110 members of Congress have signed a letter formally requesting that Mr Obama seek congressional approval for any action in Syria. 
 
 US officials are expected to give senior members of Congress a classified briefing on the evidence that the Syrian government carried out the alleged chemical attack on Thursday. 
 
 Models for possible intervention Iraq 1991: US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait 
 
 US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait Balkans 1990s: US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures" 
 
 US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures" Somalia 1992-93: UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out 
 
 UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out Libya 2011: France and UK sought UN Security Council authorisation for humanitarian operation in Benghazi in 2011. Russia and China abstained but did not veto resolution. Air offensive continued until fall of Gaddafi Syria crisis: Western military options Models for possible intervention Press apprehension as Syria tension builds Syria crisis: Where key countries stand 
 
 The US has said it will not take action alone - but one of its primary allies, the UK, has agreed to wait until UN inspectors report back before taking a parliamentary vote on potential action. 
 
 Russia rejected a UK push to try to agree a resolution on Syria among permanent UN Security Council members on Wednesday, with Foreign Minister Sergei Lavrov saying the UN could not consider any draft resolution or proposed action in Syria before the UN weapons inspectors reported back. 
 
 The use of force without a sanction of the UN Security Council would be a "crude violation" of international law and "lead to the long-term destabilisation of the situation in the country and the region", Mr Lavrov has said. 
 
 UN 'moment' 
 
 The UK, US and France are continuing their discussions following the meeting of the five permanent members. 
 
 The UK will want to be seen to be exhausting every diplomatic avenue, says the BBC's Nick Bryant at the UN headquarters in New York. 
 
 For the UK, there needs to be a UN "moment" - despite the fact that UN action will likely again be blocked by Russia or China. 
 
 But even without UN backing the US and its allies have been clear that they see the military option still open to them, our correspondent says. 
 
 "This is the first use of chemical warfare in the 21st Century," said UK Foreign Secretary William Hague. "It has to be unacceptable... or we will confront even bigger war crimes in the future." 
 
 More than 100,000 people are estimated to have died since the conflict erupted in Syria in March 2011, and the conflict has produced at least 1.7 million refugees. 
 
 Forces which could be used against Syria:
– A government backed by Russia is accused of killing its own civilians. That applies not only to the current situation in Syria but to the 1999 situation in Kosovo. And as the New York Times reports, top US officials are keeping Kosovo very much in mind as they weigh their options. President Obama would likely never get the approval of the UN Security Council for a military response because of Russia's veto power. When Bill Clinton was in the same boat in 1999, he turned to NATO and got its blessing for 78 days of airstrikes. “It’s a step too far to say we’re drawing up legal justifications for an action, given that the president hasn’t made a decision,” a government official tells the Times. “But Kosovo, of course, is a precedent of something that is perhaps similar.” Obama was meeting today with national security aides to determine next steps if the US concludes that Bashar al-Assad used chemical weapons. (Assad in turn accused the rebels of doing the same today.) Meanwhile, the US Navy is keeping two destroyers positioned in the Mediterranean in case the green light is given for a cruise-missile attack, reports the Wall Street Journal.