Paris/United Nations: Rebel groups 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 Kerry and Russian Foreign Minister Sergei Lavrov agreed on Wednesday 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, told Reuters in Damascus on Tuesday that 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.
Rebels said they would go to Munich but called on 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," Muslat told Reuters.
French Foreign Minister Laurent Fabius also questioned the commitment of the United States. "There are the ambiguities including among the actors of the coalition... I'm not going to repeat what I've said before about the main pilot of the coalition," Fabius said. "But we don't have the feeling that there is a very strong commitment that is there."
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 United States 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 IS.
De Mistura 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 and militias - 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 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.
"It'll be easy to get a ceasefire soon because the opposition will all be dead," a Western diplomat told Reuters. "That's a very effective ceasefire."
The latest fighting around Aleppo has killed about 500 people on all sides, a monitoring group said.
Other Western officials said Kerry overestimated his ability to bring Moscow around. They said he appeared to believe that since he achieved what some saw as unachievable by getting a nuclear deal with Iran he could do the same with Syria.
They noted that the two cases were different.
"The Russians are playing cat and mouse with Kerry," a senior European diplomat said.
Western officials said Moscow is clearly not committed to a ceasefire that would halt what it sees as military momentum that favours the Syrian army and its supporters.
"US policy was always based on a series of false assumptions. The main false assumption was that there is no military solution to the Syria crisis," said Christopher Harmer, an analyst with the Institute for the Study of War.
"The Assad regime has no interest in a political solution," he said. "The Russians have no interest in a political solution."
Russia says its air strikes have been targeting IS, the militant group that has seized large parts of Syria and Iraq, and not Western-backed opposition groups. But US and European officials say that is not the case.
ISSG, which includes the United States and Russia, as well as key regional powers and European nations, has been struggling to find common ground within its disparate ranks on ending the Syrian war.
After agreeing in November on a road map for a ceasefire, UN-brokered talks between the government and opposition, and eventual elections, the ISSG has been unable to move ahead with peace talks. Some officials predict that the talks, like the opposition, could die a slow death in the coming months.
A December meeting of the group was largely characterised by closed-door bickering about which groups in Syria should or should not be labelled a terrorist organisation, which would bar them from the negotiating table.
Jordan had been assigned the task of drawing up the list and collected proposals from members of the group.
The Kremlin rejects claims that it has abandoned diplomacy in pursuit of a military solution, saying it would continue to providing military aid to Assad to fight "terrorist groups" and accusing Syria's opposition of walking away from the talks.