France's Foreign Minister Jean-Marc Ayrault will visit the United Nations on Saturday to try to push through an improbable Security Council resolution to impose a ceasefire in Syria.

"I still have hope that the resolution can be approved and put into operation," he told reporters in Washington on Friday, despite a clear warning from Russia that it would veto the draft text.

"I will go to New York myself to make the case for the resolution," he added, after talks with US Secretary of State John Kerry a day after meeting Russia's Foreign Minister Sergei Lavrov.

Moscow holds a Security Council veto and its ambassador to the United Nations, Vitaly Churkin, told reporters that "I cannot possibly see how we can let this resolution pass."

The French resolution calls for an immediate ceasefire in Syria, including an end to Syrian leader Bashar al-Assad's Russian-backed assault on the rebel-held eastern half of the city of Aleppo.

Ayrault also called for a no-fly zone over the city for regime and Russian jets.

Before the talks with Ayrault, Kerry accused Moscow and Damascus of deliberately targeting hospitals.