Moscow: 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 newspaper, 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.

If Syria "follows Russia's leadership in resolving this crisis, then they have a chance to come out of it in a dignified way," Churkin stressed.

"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."

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 of the Munich agreement that "Damascus, as I hope, understands this is a unique chance for Syria after five years of unremitting destruction."

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.