The United States portrayed Bashar al-Assad's Syrian government as "dead men walking" on Wednesday, and urged Russia, China and India to put politics aside to stand with the West on action against the regime.

"Our view is that this regime is the equivalent of dead men walking," State Department special coordinator on Middle East affairs Frederic Hof told US lawmakers in a hearing on US policy toward Damascus.

In testimony to the House of Representatives foreign affairs subcommittee on the Middle East, Hof said it was clear that the regime of Assad, whose security forces have engaged in a months-long bloody crackdown on protesters, would not survive.

"It is difficult to predict how much time they have," he said.

Should Assad manage to cling to power, it would be as a despot who would "preside over a Pyongyang in Levant," crippled by sanctions much like the isolated North Korean regime, he added.

Hof also said it was vital for UN Security Council efforts to protect Syrians from Assad's oppressive forces, calling on Russia, China and India to stop opposing UN action.

"We ask those governments that are insulating this regime from the will of Syria's citizenry: do not make innocent civilians pay the price for your political calculations," Hof said.

"The international community's duty to the Syrian people transcends power politics," he testified.

"We ask that Russia, China, India and others address some basic questions: Does the regime permit peaceful protest? Does the regime allow the political opposition to organize, discuss and deliberate without fear of assassination or arrest?"

Hof said Washington must "redouble our efforts with Moscow to persuade it that it's backing of this regime is not only helping to facilitate a humanitarian catastrophe, but it is manifestly not in the interests of the Russian Federation because change is surely coming to Syria."

Washington has expressed frustration in recent weeks over what it sees as efforts by Russia and China to stymie UN action.

The two nations, like the United States both permanent members of the Security Council, used a rare double veto in October to block a Western-backed resolution condemning Assad's regime. Brazil, India, Lebanon and South Africa abstained.

The standoff intensified Tuesday when Russia slammed as "immoral" Western accusations it was blocking UN action condemning the crackdown.

Washington on Tuesday urged Russia to back UN action, and denounced Security Council silence on Syria as "unconscionable."

Hof repeated the message on Wednesday.

"If the regime succeeds in its bloody-minded effort to save itself at Syria's expense, everyone will lose. So we ask those states that have opposed UN efforts to protect Syrian civilians to reconsider," he said.

The chairman of Wednesday's hearing, Republican Representative Steven Chabot, blasted President Barack Obama's administration for waiting so long before calling in August for Assad's departure, but also slammed the Russia-China veto as "outrageous and indefensible."

And while Chabot said the policy of encouraging non-violence against the regime "grows more untenable," Hof responded that he was not calling for Syrians to embrace violence.

"Syrians across the board recognize that the cost of a fight to the finish will be prohibitive," he said.

The United Nations estimates more than 5,000 people have been killed in the crackdown since March.