It is a "mistake to prop up that regime," President Barack Obama's spokesman Jay Carney told a White House briefing shortly after Russia and China blocked UN action against Syria for the third time in nine months.
The veto is a "highly regrettable decision that will, I think, have repercussions for the countries that vetoed the resolution for a long time, in terms of how they're viewed by the Syrian people," Carney said.
"There's no doubt that Syria's future will not include Bashar al-Assad. His days in power are numbered. It's a mistake to prop up that regime as it comes to an end."
Carney's reaction followed that of US Ambassador to the United Nations Susan Rice, who said bluntly that the UN Security Council had failed on Syria.
"We will intensify our work with a diverse range of partners outside the Security Council to bring pressure to bear on the Assad regime and to deliver assistance to those in need," Rice said.
"The Security Council has failed utterly in its most important task on the agenda this year," Rice added, as she slammed Moscow and Beijing.
Highlighting Syria's stockpile of chemical weapons, Rice said the Syrian government will be "held accountable" if it is used against the opposition.
"As the situation deteriorates, the potential that this regime could consider using chemical weapons against its own people should be a concern for us all," she said.Â Fears over the chemical weapons have increased in recent days.
On Monday, Nawaf Fares, who defected from his post as Syrian ambassador to Iraq, said in an interview with the BBC that he was "convinced" that Assad would draw on his stocks if cornered.
The latest Russia-China veto deepened an acrimonious battle at the 15-nation council over who is to blame for the world powers' failure to get international action to halt the Syria conflict.
There were 11 votes in favor, with Russia and China voting against and with Pakistan and South Africa abstaining. As two of the five veto-wielding permanent members of the council, Russia or China can block any resolution.