US Secretary of State Hillary Clinton has dismissed a Russian proposal for a new UN Security Council resolution on Syria as pointless "without teeth".
Russia says it wants Security Council approval for a peace plan agreed in June in Geneva that called for a ceasefire and political transition.
But Mrs Clinton said a resolution without consequences would be ignored by Syrian President Bashar al-Assad.
Activist groups in Syria say more than 160 people died in the latest fighting.
Government air attacks are said to have killed many in the northern city of Aleppo, where a water supply pipe was also hit.
Meanwhile, state TV has shown government troops overrunning rebel-held areas in the south of the capital Damascus.
It also showed them being welcomed by some of the residents, who chanted in support of the army and the regime.
But the BBC's Jim Muir, who is in neighbouring Lebanon, says the fighting in Damascus and its suburbs has been going on for nearly two months now, with rebels reappearing after government forces move on.
He adds that the same holds for Aleppo, where even the heavy use of air strikes as well as artillery bombardments have failed to dislodge the rebel fighters.
'Realistic'Speaking at the end of the Asia-Pacific Economic Co-operation (Apec) summit in the Russian city of Vladivostok, Mrs Clinton was frank about the gulf between the US and Russia on Syria, a traditional Moscow ally.
"We have to be realistic. We haven't seen eye-to-eye... that may continue," she told reporters after talks with Russian Foreign Minister Sergei Lavrov and President Vladimir Putin.
"There is no point to passing a resolution with no teeth because we've seen time and time again that Assad will ignore it and keep attacking his own people.
"I will continue to work with Foreign Minister Lavrov to see if we can revisit the idea of putting the Syrian transition plan that we agreed to in Geneva earlier this summer into a Security Council resolution.
"But as I underscored yesterday... that will only be effective if it includes consequences for non-compliance."
She said that if differences persisted, the US would "work with like-minded states to support a Syrian opposition to hasten the day when Assad falls, and to help prepare Syria for a democratic future and help it get back on its feet".
Our correspondent says that with more than 100 people killed even on an average day, international diplomacy remains unable to get a grip on the crisis.
Talks at the Pacific Rim meeting have served only to entrench differences between the Russians and Americans that have paralysed the Security Council, he adds.
The new UN-Arab League special envoy, Lakhdar Brahimi, is due in the area shortly, but he has admitted he has yet to formulate a peace plan that might work.
Activists say at least 23,000 people have been killed in the 18 months since the start of the conflict.
