Russia had "very clearly and consistently explained" that it would not accept a resolution that "would open the path for pressure of sanctions and further to extend military involvement in Syrian domestic affairs," said Vitaly Churkin, Russia's ambassador to the United Nations.
Western diplomats "could have done something, anything, to promote dialogue" rather than "fan the flames of extremists," Churkin said. He described the draft resolution as "biased," saying it threatened sanctions exclusively at the Syrian government.
Susan Rice, U.S. ambassador to the United Nations, told CNN's "The Situation Room" that the United States will push other approaches to the conflict, including through the Friends of Syria Group.
"What Russia and China did today by vetoing a third resolution was really to slam the door, at least in the short term, on any effective action out of the U.N. Security Council," she said.
She called the two countries "isolated outliers" on the issue.
"History will judge them as having stood by a brutal dictator at the expense of his own people and at the expense of the will of the international community and the countries in the region," Rice told CNN.
The fate of the U.N. observer mission was unclear after the vetoes.
Competing drafts, sponsored by the United Kingdom and Pakistan, are expected to be considered by the U.N. Security Council. They highlight the simmering differences among members.
The UK mission said its resolution will be considered Friday morning. That draft measure extends the observer mission in Syria for a final period of 30 days.
Russia, which backs Pakistan's resolution, wants observers to stay in the country and promote dialogue. Western countries are concerned about the safety of observers and want one final deployment.
China's U.N. ambassador, Li Baodong, said his country is "highly concerned" about the violence, but Thursday's draft resolution was "seriously problematic," because it intended to pressure only one side in the conflict. It would not help resolve the issue "but derail the issue from the track of political segment," further aggravating turmoil, he argued.
Syria's U.N. ambassador, Bashar Jaafari, slammed the council for not issuing an official condemnation of Wednesday's bombing, although he noted that Kofi Annan, joint envoy to Syria for the United Nations and the Arab League, condemned it.
The lack of an official condemnation by the council indicates that talk of "supporting a peaceful solution in Syria is but a slogan," Jaafari argued.
He rejected a "misleading picture" of Syria as a tyrannical regime killing its people and said a solution should be found through a "political process that satisfies the Syrian people." All will participate to help "establish democracy" and "free elections," he insisted.
The al-Assad family has ruled Syria for more than 40 years.
Residents trying to flee the Syrian violence have nowhere to go because clashes are raging in most of Damascus province, said Omar al-Dimashki, a spokesman for the Revolution Leadership Council of Damascus.
"It is surrounded by tanks, and anything that moves is currently being shelled, and rockets are falling on the homes," he said.
Sander van Hoorn, a journalist in Damascus with Dutch TV network NOS, a CNN affiliate, said, "One hour it can be quiet and the next hour all hell breaks loose."
A resident of the al-Qaboun neighborhood in Damascus said he fled with his wife and two sons after an explosion shook a house next door.
The man, who CNN is not identifying for security reasons, said his sister and her family joined them in Qamishli, and she described the government assault on al-Qaboun. "There were tanks, helicopters, armored personnel carriers and gunshots fell like rain. We were terrified," he quoted her as saying.