US Secretary of State John Kerry on Tuesday said he would try to get Syrian peace talks going again with Russia's help as government forces closed in on rebels in the besieged city of Aleppo.

Moscow launched an air campaign last year to help long-time ally Syrian President Bashar al-Assad who now appears on the verge of capturing all of Aleppo in what would be a huge blow to the rebel cause.

"We have been trying to find a way to get to the negotiating table ... but Assad has never shown any willingness," Kerry said at a NATO foreign ministers meeting in Brussels.

"Russia says Assad is ready to come to the table ... and I am in favour of putting that to the test," he said.

Kerry, who has had repeated meetings on Syria with his Russian counterpart Sergei Lavrov, said he would have another discussion with him when the two meet later this week.

Even if Aleppo falls, "the war will continue," Kerry said, stressing that there had to be a negotiated settlement.

Talks between the Syrian government and western-backed rebels have made no progress, with both sides accusing each other of bad faith.

Kerry had earlier rejected charges by Lavrov that Washington was stalling talks on a rebel withdrawal from Aleppo in order to buy them time.

Moscow had the impression "a serious discussion with our American partners isn't working out," Lavrov said.

Lavrov on Monday had said Russia and the US would meet in Switzerland on either Tuesday or Wednesday to discuss a proposal put forward by Kerry last week on a complete rebel withdrawal from eastern Aleppo.

The rebels dismissed any withdrawal, prompting Lavrov to warn that "they will be destroyed" if they stay.

Russia and China on Monday vetoed a UN Security Council resolution calling for a seven-day ceasefire in Aleppo.

Source: AFP