A senior figure in the Syrian opposition movement on Sunday criticised the truce deal forged by the US and Russia, saying Moscow was continuing its onslaught on civilian areas.

"We have gotten used to conferences and hope put into words but what we need is action, and the action I see is that Russia is killing Syrian civilians," said Riad Hijab, head of the High Negotiation Committee that represents several Syrian opposition groups.

"The Syrian people continue to live in terror and utter despair after the international community has failed to prevent the gravest crimes," he told the audience at the Munich Security Conference.

Hijab dodged questions about whether the "moderate" rebels would accept the "cessation of hostilities" agreement reached on Friday that calls for a truce within a week.

"Why is the onus on the opposition and whether it has preconditions for negotiations? I would like to see a single day of a cessation of hostilities in order to give a chance for real political movement," said Hijab.

Critics have said Friday's deal is hobbled by the fact it does not include "terrorist" groups such as the Islamic State group and the Al-Qaeda affiliate Al-Nusra, leaving room for Russia to continue attacks by claiming it is targeting jihadists.

It followed a major offensive by Syrian government forces, backed by heavy Russian bombing and Iranian troops, on the rebel stronghold of Aleppo.

- 'Very pessimistic' -

Others, including Israeli Defence Minister Moshe Yaalon, lined up to voice their doubts about the deal.

"I'm very pessimistic about a potential cessation of hostilities at the end of this week. Unfortunately we are going to face chronic instability (in Syria) for a very, very long period of time," said Yaalon.

He said it was very hard to imagine Syria being reunited.

"We know how to make an omelette from an egg, we don't know how to make an egg from an omelette. We are going to see enclaves -- Alawi-stan, Syria-Kurdistan," said Yaalon.

US Senator John McCain also slammed the deal, saying it would only empower Moscow's "military aggression".

"Let's be clear about what this agreement does: It permits the assault on Aleppo to continue for another week. It requires opposition groups to stop fighting but it allows Russia to continue bombing terrorists which it insists is everyone, including civilians," said McCain, a leading member of the opposition Republicans and head of the Senate Armed Forces Committee.

"If Russia or the Assad regime violates this agreement, what are the consequences? I don't see any," he told the conference.

McCain said it was "no accident" that Russian President Vladimir Putin had chosen this moment for a deal.

"We've seen this movie before in Ukraine," he said. "Russia presses its advantage militarily, creates new facts on the ground, uses the denial and delivery of humanitarian aid as a bargaining chip, negotiates an agreement to lock in the spoils of war and then chooses when to resume fighting."