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."

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.

Still, one of the officials said, "This remains a campaign to apply economic and diplomatic pressure." Neither official was willing to be identified because neither is authorized to talk to the news media.

Against the backdrop of talk about possible plans for military intervention, the bloodshed continued unabated in Syria.

In Homs, an opposition activist CNN will identify only as "Danny" for his safety, said government soldiers had been going house to house by breaking through walls rather than entering the street, where they might be targeted by members of the Free Syrian Army, comprising soldiers who have defected from pro-government forces. "They've been bombarding us from 5 a.m., like yesterday, with rockets," Danny said.

Soldiers had been shooting mortars and rockets into the neighborhood, he said. Among the targets on Tuesday was a Red Cross ambulance, he said. The soldiers have taken up residence in the lone hospital in the southwest Homs neighborhood of Baba Amr, he said. "They hit the operation room while doctors are doing operations; they kicked the doctors out of the hospital and all of the nurses out of the hospital and left all of the people in there."

Civilian casualties who seek treatment there won't find it, he said. "You're taken by security forces and tortured to death or they let you bleed to death," he said. Instead, casualties are seeking treatment at makeshift hospitals set up in private buildings, he said.

Such accounts appear to have had an impact thousands of miles westward, in Washington. There, U.S. Sen. John McCain, a leading Republican and ranking member of the Senate Armed Services Committee, said Tuesday that 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."

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 discussion came a group of Arab nations announced they are calling home their ambassadors. Among them was Bahrain, which last year had its own robust crackdown on dissidents.

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 statement.

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.

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.