By Tom Miles
GENEVA (Reuters) - The United States is failing to stand up to Russia, which is committing "war crimes" in Syria, a leading Syrian opposition negotiator said on Sunday, as Russian-backed forces appeared to tighten the siege on the city of Aleppo.
"What we lack here is a serious reaction to Russian behavior on the ground," said Basma Kodmani, a member of the main opposition High Negotiations Committee.
"Russia is saying one thing and doing another," she told reporters on a conference call.
The chances of a new round of peace talks were looking increasingly remote as Russia participated in airstrikes after "constantly lying" about what it was willing to do for peace in Syria, she said.
The United Nations had said talks between U.S. Secretary of State John Kerry and Russian President Vladimir Putin might bring an understanding on the two biggest obstacles to a return to peace talks - agreement on stopping indiscriminate bombing and a formula for a political transition.
But Kerry ended his trip to Moscow by saying that although there was a common understanding on the steps needed to get the peace process back on track, more work was needed before those steps could be implemented.
"What we need is for Russia to restate that it is interested in a political process. We don't see that," Kodmani said.
"This is where we expect the Americans to react more forcefully, because these are war crimes being committed at the very moment when Mr Kerry is in Moscow and is discussing a security arrangement and the targeting of terrorist groups, while Russia is fully engaged in this operation on Aleppo."
Syria enjoyed two months of widespread peace thanks to a "cessation of hostilities" that began in February. But it has all but collapsed and Kodmani said efforts to restore it had "failed miserably".
"We would like to see what kind of guarantees the U.S. is able to agree with Russia," she said. "What we want is solid guarantees about the cessation of hostilities being restored."
The United States and its allies in Europe and the Middle East must be able to take steps to weigh in more heavily to challenge Russia, but it appeared there were no such measures, she said.
"It really is quite baffling to see that the Western powers who are supposed to be the friends of the Syrian people are not putting any alternative approach. There's a big void out there."
(Reporting by Tom Miles)