'Your days are numbered,' U.S. official tells al-Assad NEW: "Scoping exercise" under way to assess U.S. military capability in region, officials say They may begin building a case for the International Criminal Court, a source says UNICEF cites "deaths and injuries of hundreds of children" Sen. John McCain says the United States should consider arming the Syrian opposition (CNN) -- As violence raged in Syria, U.S. officials made clear Tuesday that the United States has lost patience with President Bashar al-Assad. "Your days are numbered," U.S. Ambassador to the United Nations Susan Rice said in comments directed at the Syrian leader. It is time and past time for you to transfer power responsibly and peacefully. In an interview with CNN, Rice said, "Before we start talking about military options, we ... want to make sure we've exhausted the diplomatic methods at our disposal." Her remarks came as two senior administration officials told CNN that, while the U.S. focus remains on exerting diplomatic and economic pressure on Damascus, the Pentagon and U.S. Central Command have begun a preliminary internal review of U.S. military capabilities in order to prepare options in the event that President Obama calls for them. One of the officials called the effort a "scoping exercise" to see what capabilities are available given other U.S. military commitments in the region. Both officials said this type of planning exercise is typical for the Pentagon. The Pentagon is closely monitoring developments in Syria. It wouldn't be doing its job if it didn't put some ideas on the table," one of the officials told CNN. But absolutely no decisions have been made on military support for Syria. Neither official was willing to be identified because neither is authorized to talk to the news media. In such an analysis, the military would typically look at all options, from humanitarian relief to support for opposition groups, as well as outright military strikes, although the latter is unlikely, the officials said. "This remains a campaign to apply economic and diplomatic pressure," the first official said. Also Tuesday, U.S. Sen. John McCain, a leading Republican and ranking member of the Senate Armed Services Committee, said the United States "should start considering all options, including arming the opposition. The bloodletting has got to stop. White House press secretary Jay Carney said the United States has no immediate plans to arm the opposition. "We are not considering that step right now," he told reporters. We are exploring the possibility of providing humanitarian aid to Syrians. And we are working with our partners, again, to ratchet up the pressure, ratchet up the isolation on Assad and his regime. GPS: Al-Assad hangs on by being ruthless Carney added that U.S. officials were "seeing a lot of indications of a lack of control over the country by the regime, of interest by senior officials within the military and the government in separating themselves from the regime." But State Department spokeswoman Victoria Nuland did not rule out the possibility of arming the opposition. "We never take anything off the table," she told reporters. However, as the president himself made absolutely clear and as the secretary has continued to say, we don't think more arms into Syria is the answer. We think the answer is to get to a national democratic dialogue, for the violence to stop, for the regime's tanks to come out of the cities, and then for monitors to be able to go back in. The heightened discussion came as images on social media showed children dead in the streets and a group of Arab nations announced they are calling home their ambassadors. Wolf Blitzer: This is an international disaster The Gulf Cooperation Council said it made the decision "with deep sorrow and anger" at the increased pace of killings in Syria "that did not spare a child, old man, or woman -- heinous acts that can be described as a collective massacre against the defenseless Syrian people without any mercy or pity, and without considering any rights or feelings of humanity or morality." The United Nations' Children's Fund said there have been "deaths and injuries of hundreds of children." "There are reports of children being arbitrarily arrested, tortured and sexually abused while in detention," the organization said in a written statement. On the ground in Syria: "For God's sakes, this is too much" The six Gulf Cooperation Council nations -- the United Arab Emirates, Bahrain, Saudi Arabia, Oman, Qatar and Kuwait -- also expelled Syria's ambassadors, saying "their presence became useless after the Syrian regime has rejected all attempts, and thwarted all Arab sincere efforts to resolve this crisis and stop the bloodshed of the Syrian people." Spain, France and Italy called home their ambassadors as well. Britain did so on Monday, the same day the United States closed its embassy in Damascus, saying the Syrian government was refusing to address its security concerns. The Gulf Cooperation Council also called on Arab League nations to "take decisive action against this dangerous escalation against the Syrian people" at a meeting next week. The council may pursue another step. Use of the term "collective massacre" in the statement indicates the members are ready to begin building a case against the Syrian regime for consideration by the International Criminal Court, a council diplomatic source told CNN. By calling for "decisive action," the council is pressuring members of the Arab League, such as Sudan, Algeria and Iraq, to withdraw their support for the al-Assad regime and facilitate more aggressive decisions, the source said. Syria's terrible pain, told in photos Meanwhile, Syria tried to project an image of support for the regime. Throngs of supporters shown on state TV cheered the arrival of Russian Foreign Minister Sergey Lavrov for a meeting with al-Assad in Damascus, three days after Russia and China vetoed a U.N. Security Council resolution that would have demanded al-Assad stop the violence against the opposition in his country. Every leader in every country should understand their responsibility. You understand yours," Lavrov told al-Assad, according to Russia's state-run Ria Novosti news agency. It's in our interests that the Arab peoples live in peace and harmony. In a statement reported by the state-run news agency SANA, Lavrov said al-Assad affirmed his commitment to the Arab League plan "of putting an end to violence regardless of its source. To this end, Syria affirms its interest in continuing work with the Arab League observer mission and increasing observer numbers to cover all points and verify any breaches or violations to the principle of not allowing violence regardless of source. Asked about Lavrov's visit, White House press secretary Carney said, "Well, we're not sure what the goals of his visit are. But the point we're making is that Russia must realize that betting everything on Assad is a recipe for failure, not just for Russia's interests in Syria but for the stability of the region and for Syria's future. But in an interview Monday with PBS's Charlie Rose, Russia's U.N. ambassador, Vitaly Churkin, said Moscow had been trying to work with Western countries, including the United States, to stop the killing in Syria. Opinion: World must aid Syria's rebels "Unfortunately, from the outset they decided that it was a regime-change opportunity, so from the outset they were announcing that Assad has lost his legitimacy," Churkin said. The Arab League suspended its observer mission because of the sharp increase in violence. The Syrian government, which routinely blames the violence on "armed terrorist groups," said such groups were behind violence in parts of the country Tuesday. "Terrorist groups continued armed attacks in Homs, Idlib and Damascus countryside targeting civilians and military personnel, and shelled residential areas with mortar shells and vandalized public and private facilities, not sparing even mosques to which citizens turned for protection," SANA reported. The Syrian Interior Ministry, in a statement on state TV, vowed that "the operation of pursuing the terrorist groups will continue" until security is restored in Homs. The mission includes "terminating anyone who carries a weapon and is terrorizing the citizens," the statement said, adding that "the opportunity is still there for members of these groups to surrender." At least 35 people were killed Tuesday in Syria, including 19 in Homs, the city that has seen some of the worst violence in recent days; 10 in Damascus suburbs; two each in Idlib; Daraa and Aleppo, according to the opposition Local Coordination Committees of Syria. Among the dead were a father, mother and their three daughters, ages 24, 16 and 12, LCC said. Separately, the Syrian Observatory for Human Rights, a London-based opposition group, said late Tuesday that new clashes had erupted between defectors and security forces at a checkpoint in the city of Daeel in Daraa province. "The entire world should be ashamed of what's happening here," a Damascus-based opposition member told CNN. "Children are hungry," and people are without power, said the activist, identified only as Zaidoun for his safety. He added, "Everybody is just silent and looking at us being slaughtered every moment, for no reason -- just for asking for our freedom." Several countries issued new calls for Syria to halt the violence. Turkish Prime Minister Recep Tayyip Erdogan had some of the sharpest words, calling on al-Assad "to return from this wrong path before taking the lives of more innocent civilians." In comments broadcast live on Turkish TV, Erdogan said he had a message for al-Assad "in a way he can understand: ... What goes around comes around. Erdogan also called events at the U.N. Security Council "a fiasco in terms of the civilized world." Opposition activists and several of the 13 U.N. Security Council member nations that voted for the resolution have said Russia and China have blood on their hands for vetoing it as more members of the opposition are killed. Lavrov described Western condemnation of the veto as "hysterical," according to Ria Novosti. Both China and Russia have said they oppose the violence and want the situation resolved through dialogue. "We believe in continuing the efforts to reach a resolution for the Syrian crisis," Syrian state TV quoted Lavrov as saying. Russia needs peace and an agreement has to be reached, away from any foreign interference. While world leaders seek ways to mount pressure on the regime, the situation inside Syria "is beyond description," according to the opposition Syrian Revolution General Commission. The group reported 128 deaths Monday, mostly in Homs. "Some of these martyrs were killed with shrapnel and the others were under the rubble, and their bodies couldn't be identified because they were in remains," the group said. Mousa Azzawi of the Syrian Network for Human Rights, speaking to CNN in London, said, "The situation is very dire," and Monday was almost "like a bloodbath." We have pictures of children under the age of 14 with half of their faces blown away; with children under the age of 4 with all of their bodies with nail bombs. U.N. officials estimate 6,000 people have died since protests began nearly a year ago. But an article published Monday in The Times of London said Asma al-Assad appears to support her husband, while also seeking dialogue and comfort for the bereaved in the country. According to an e-mail sent through an intermediary from Asma al-Assad's office, "The president is the president of Syria, not a faction of Syrians, and the first lady supports him in that role," The Times reported. "The first lady's very busy agenda is still focused on supporting the various charities she has long been involved with and rural development as well as supporting the president as needed," the e-mail continued, according to The Times. Protesters and rebel fighters are demanding an end to al-Assad's rule and democratic elections. An opposition activist in Homs, identified only as "Danny" to protect his safety, said government forces fired rockets on the city after the U.N. draft resolution failed over the weekend. I'm next to a window and a rocket might kill me," he said. CNN's Barbara Starr, Kamal Ghattas, Arwa Damon, Azadeh Ansari, Jennifer Deaton, Mitra Mobasherat, Elise Labott, Mick Krever, Richard Roth and Livia Borghese contributed to this report.