Russia's envoy to the UN on Friday warned long-term ally President Bashar al-Assad over his vow to retake all of Syria, saying he faced dire consequences if he did not comply with Moscow over the peace process.

"Russia has invested very seriously in this crisis, politically, diplomatically and now also militarily," Vitaly Churkin told Kommersant daily, referring to an international agreement to cease hostilities sealed in Munich last week.

"Therefore we would like Assad also to respond to this," he said, adding that the Syrian leader's stance "is not in accord with the diplomatic efforts that Russia is making."

At their meeting in Munich, the 17-nation group backing Syria's peace process agreed to work for a ceasefire, the lifting of starvation sieges and the resumption of talks.

In an interview with AFP last week, Assad defiantly pledged to retake the whole of the country, speaking before the plan for a nationwide "cessation of hostilities" in Syria was announced.

Asked to comment to journalists on the unusually outspoken criticism of Assad, Kremlin spokesman Dmitry Peskov on Friday said President Vladimir Putin backed the Syrian peace process but stressed that the ceasefire had not yet been implemented.

"Everyone including President Putin recognises that there is no alternative other than a political resolution," he said.

Nevertheless the ceasefire "is now being worked out, discussed. Wait, let's not run ahead," Peskov said.

In the interview, Churkin, who has served as Russia's ambassador to the UN since 2006, stressed that if Syria "follows Russia's leadership in resolving this crisis, then they have a chance to come out of it in a dignified way."

"If they in some way stray from this path - and this is my personal opinion - a very difficult situation could arise. Including for themselves," he warned.

"If they proceed on the basis that no ceasefire is necessary and they need to fight to a victorious end, then this conflict will last a very long time and that is terrifying to imagine."

Syria is "already on the brink of falling apart," he said.

'Too late' for no-fly zone  

Churkin however also suggested that Assad's comments were made for political impact.

"It isn't worth putting too much significance into one statement or another and dramatising them," he said.

"We should be guided not by what he says, with all respect for the statements of a person at such a high level, but by what he finally does."

Churkin said he hoped that Damascus realised that the Munich agreement was "a unique chance for Syria after five years of unremitting destruction."

While German Chancellor Angela Merkel has repeated calls for a no-fly zone in Syria to protect civilians, Churkin dismissed this as unworkable.

"It's already too late to talk about this. When everyone is flying and bombing, there is no question of no-fly zones," he said.

He said it was "theoretically possible" to imagine such a proposal during the process of implementing the ceasefire agreement but called this "purely theoretical... particularly in the circumstances of our presence in Syria."

Russia launched air strikes in Syria in September last year to support Assad and fight "terrorists", saying it was targeting the Islamic State group and other jihadists.