White House spokesman Jay Carney backed a Red Cross call for a daily cease-fire in Syria in order to deliver humanitarian aid.
"The reprehensible actions perpetrated by the Syrian regime, the brutal violence perpetrated by the Syrian leader against his own people, has led us to this situation where basic supplies, humanitarian supplies are very scarce and therefore action needs to be taken," Carney said.
U.S. State Department spokeswoman Victoria Nuland said Washington was focused on "increasing the international isolation and the international pressure on the Assad regime to stop the violence altogether, so that we can move on to a democratic transition."
In the northern province of Aleppo, the government said a Syrian businessman, Mahmoud Ramadan, was shot to death in front of his home in what appeared to be the latest in a series of targeted killings. The attacks, which include the slaying of an Aleppo city council member Saturday, suggest that rebel factions are increasing turning to arms to strike back at members of Assad's ruling system.
Residents and activists say a monthslong siege and stepped up attacks on Baba Amr recently have left the district without enough food, water, medicine and electricity.
"They bombed all the water tanks on the roofs of buildings. There's no water. Some people have gone without bread for days," said Shaker, who estimated the shells fell at a rate of about 10 per minute at some points in the attack. More than 200 people were wounded, he said, adding that two children were among the dead.
Phone lines with Homs have been cut, making it difficult to get firsthand accounts from residents.
One amateur video posted on the Internet showed thick smoke and shells slamming behind a building in Baba Amr. Another showed a shop on the ground floor of a building on fire as a narrator cries: "We are dying. Where are the Arabs?"
The Arab League has tried to pressure Assad into a peace process with the opposition, but he has refused.
In another possible shift away from Assad, about 500 Palestinians gathered in Gaza at a Hamas-authorized demonstration in solidarity with Syrian protesters.
Assad has long hosted and supported leaders of the Islamic Hamas movement, which rules Gaza. But as the body count in Syria continues to rise, Hamas has been trying to distance itself from Damascus. Hamas has forged closer ties with rich Gulf states that oppose the Syrian regime and seeks to undercut Iran's influence.
A planned international meeting later this week in Tunisia will seek ways to help the Syrian people.
"People don't care if it's the devil intervening to save us from Bashar. We need the world's help," Shaker said.
In Moscow, Russian Foreign Ministry spokesman Alexander Lukashevich said Tuesday it will not attend the planned "Friends of Syria" meeting because organizers did not invite Syrian government representatives.
Russia and China have vetoed two U.N. Security Council resolutions backing Arab League plans aimed at ending the conflict and condemning Assad's crackdown on protests that killed 5,400 people in 2011 alone, according to the U.N. Hundreds more have been killed since, activist groups say. One of the groups puts the toll at more than 7,300.
Lukashevich said the meeting wouldn't help a dialogue, saying that the global community should act as friends of all the Syrian people, not just one part.
"It looks like an attempt to forge some kind of international coalition like it was with the setting up of a 'Contact Group' for Libya," he said.
Russia has said it will block any U.N. resolution that could pave the way for a replay of what happened in Libya. In that case, Russia abstained from a vote, which cleared the way for months of NATO airstrikes that helped Libyans end Moammar Gadhafi's regime.
In Jerusalem, Sen. John McCain condemned Russia and China for vetoing sanctions against Syria, saying their action was "not the behavior of mature nations." He suggested that weapons should be sent to those fighting the regime.