The United States has dismissed a proposal by Russia for a 1 March ceasefire in Syria, saying that the Kremlin is giving itself and the Assad regime a three-week window in which to crush moderate rebel groups.

Washington countered the proposal with demands for the fighting to stop immediately, US officials said. Peace talks are supposed to resume by February 25.

The talk of new ceasefire plans comes as the US, Russia and more than a dozen other countries meet in Munich to try to halt five years of civil war in Syria.

The most recent Russian-backed offensive, near Aleppo, prompted opposition groups to walk out of peace talks last month in Geneva, while forcing tens of thousands of civilians to flee towards the Turkish border.

One US source said America could not accept Russia's offer because opposition forces could suffer irreversible losses in northern and southern Syria before the ceasefire even took hold.

The officials said the US counter-proposal was a simple ceasefire effective immediately and accompanied by full humanitarian access to Syria's besieged civilian centres.

The conflict has so far killed more than a quarter of a million people, created Europe's biggest refugee crisis since the Second World War and allowed the Islamic State to carve out its own territory across parts of Syria and neighbouring Iraq.