US, Russia achieve 'clarity' on path to new Syria ceasefire: Kerry

Washington and Moscow have "achieved clarity" on the way towards a new ceasefire in Syria, but technical details must still be resolved, US Secretary of State John Kerry said on Friday.

"Today I can say that we achieved clarity on the path forward" towards a revamped cessation to the fighting, Kerry told reporters alongside his Russian counterpart Sergei Lavrov after marathon talks.

Lavrov added that "substantial agreements had been reached", while noting that "some things need to be finalised".

Kerry stressed that the two sides were not prepared to make a definitive announcement, but said US and Russian technical experts would continue to meet in Geneva in the coming days to hammer out a set of unresolved issues.

"We are working on those issues," Washington's top envoy said. "Neither of us is (ready) to make an announcement that is predicated by failure - we don't want to have a deal for the sake of a deal."

Kerry linked the near total collapse of a previous ceasefire agreed earlier this year to the failure to respond to violations and the Syrian regime's efforts to reclaim strategic territory, including around Damascus.

Successive rounds of international negotiations have failed to end a conflict that has killed more than 290,000 people and forced millions from their homes.

Moscow and Washington support different sides in the war, which erupted in 2011 after President Bashar al-Assad unleashed a brutal crackdown against a pro-democracy revolt.

Friday's meeting came as the conflict became further complicated by Ankara's decision this week to send tanks into Syria.

Turkish-backed rebel fighters have seized the Syrian border town of Jarabulus from Islamic State (IS) group militants. But Turkish forces have also shelled a US-backed Syrian Kurdish militia.

Turkey sees the PYD and YPG militias as terror groups bent on carving out an autonomous region in Syria and acting as the Syrian branch of its own outlawed Kurdish PKK.

Ankara's hostility to the YPG also puts it at loggerheads with its NATO ally the United States, which works with the group on the ground in the fight against IS.

The Russian air force has been carrying out air strikes in Syria since September last year, claiming it only targets militants.

The West and the Syrian opposition have accused it of hitting civilian targets in rebel-held areas - claims that Moscow denies.

But the US and Russia have a common foe in IS, and they have been in contact on efforts to establish military cooperation against the group.

As a possible sign of increasing cooperation, Moscow vowed on Thursday to work with the US on a response after a UN investigation found that the Syrian government had carried out at least two chemical attacks.

The two countries co-chair a UN-backed humanitarian taskforce for Syria, which has been struggling to ensure access for desperately-needed aid across the country.

Aleppo, Syria's second city, has emerged as a top concern for negotiators.

Russia last week gave its blessing to a long-demanded 48-hour pause in fighting in the city to allow in aid, but de Mistura on Thursday accused other unspecified parties of dragging their feet.