Rebel groups have urged US President Barack Obama to do more to stop Russian bombing raids in Syria as pressure mounted on Washington for greater commitment towards resolving the five-year-old war ahead of a new round of peace talks this week.

World powers are meeting in Germany on Thursday in a bid to revive peace efforts but with Moscow backing a Syrian government push for all-out military victory, Western officials and opposition delegates see little hope of a breakthrough.

UN Special Envoy Staffan de Mistura halted the first attempt to negotiate an end to Syria's war in two years after an offensive by forces loyal to Syrian President Bashar al-Assad against Western-backed rebels backed by Russian air strikes.

Trying to prevent a collapse of diplomatic efforts to end the war, US Secretary of State John Kerry is pushing for a ceasefire and more humanitarian aid access ahead of a meeting of the International Syria Support Group in Munich.

Moscow said Mr Kerry and Russian Foreign Minister Sergei Lavrov had agreed on the need for a ceasefire in Syria and the provision of humanitarian aid to blockaded areas.

But one UN diplomatic source said Russia was "stringing Kerry along" in order to provide diplomatic cover for Moscow's real goal - to help Assad win on the battlefield instead of compromising at the negotiating table.

"It's clear to everyone now that Russia really doesn't want a negotiated solution but for Assad to win," said the diplomatic source, speaking on condition of anonymity.

A senior adviser to Assad, Bouthaina Shaaban, said there would be no let-up in the army advance, which aimed to recapture the city of Aleppo from rebels and secure Syria's border with Turkey.

Saudi-backed rebels said they would go to Munich but called on Mr Obama to be more forceful with Russia over its bombing campaign. The rebels would only go to UN peace talks in Geneva later this month if Russia stops bombarding their positions and humanitarian aid reaches civilians in the areas they control.

Spokesman Salim al-Muslat said: "I believe he (Obama) can really stop these attacks by the Russians on Syrians. If he is willing to save our children it is really the time now to say 'no' to these strikes in Syria."

"I believe he can do it but it is really strange for us that we don't hear this from him."

Rebel groups say that while Washington has put pressure on them to attend peace talks, they see less help on the battlefield. Appeals for anti-aircraft missiles to counter the latest offensive are falling on deaf ears.

Turkey, meanwhile, upbraided the US for supporting Syrian Kurdish PYD rebels, saying Washington's inability to understand the group's true nature had turned the region into a "sea of blood".

"Are you on our side or the side of the terrorist PYD?" President Tayyip Erdogan said, referring to Washington's backing of the group against Islamic State.

De Mistura has set a target date of February 25 to reconvene talks between the Syrian government and opposition in Geneva.

But in less than two weeks, the offensive by Syrian forces, Hezbollah and Shiite militias directed by Iran - all backed by Russian bombing raids - have reversed opposition gains on the ground and encircled rebels inside Aleppo, a strategic prize now divided between government and opposition control.

This has caused alarm among UN and Western officials, who believe the goal of the Syrian-Russian-Iranian campaign is to destroy the opposition's negotiating power in Geneva, kill them on the ground, and secure the first major military victory since Moscow began bombing opposition forces in Syria in September.