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UN mediator Staffan de Mistura on Thursday called on Moscow and Washington to push for a resumption of Syrian peace talks, which he hoped could resume next month.

De Mistura told reporters in Geneva he wanted to "ensure that we have enough, sufficient, critical mass in order to give a sufficient chance for a concrete and effective beginning of the third round of the intra-Syrian talks."

The talks "have a target date of August," de Mistura said.

"These talks ... are meant to be a credible beginning of a roadmap towards a political transition, so they need to be well-prepared," he said.

De Mistura, who had previously spoken about resuming the talks in July, made his comments as US Secretary of State John Kerry was on his way to Moscow for talks with President Vladimir Putin and test his commitment to the stalled Syrian peace process.

Washington and Moscow have backed a roadmap that calls for a nationwide ceasefire and Geneva-based talks on "political transition."

There has been little progress towards a hoped-for resumption of talks this month

But there has been little progress towards a hoped-for resumption of talks this month, and the prospects for a political transition beginning by August, as laid out in the road map, now appear slim.

De Mistura pointed to the partial ceasefire that Moscow and Washington brokered for Syria in February, saying that when the two "agree on something ... that helps a lot the political process and even the humanitarian process."

De Mistura said he hoped Washington and Moscow could help reduce the "non-constructive ambiguity" surrounding al-Nusra Front.

Like the Islamic State group, the Nusra Front is defined as a terrorist group by the UN Security Council, but is not party to the much-breached ceasefire.

That ambiguity, he said, "has been one of the main problems for the sustainability of the cessation of hostilities."

While until now all sides have agreed on targeting IS, Al-Nusra -- an affiliate of al-Qaeda -- has been more tricky since it is mainly battling Assad alongside other rebel groups backed by US allies.

The Washington Post on Thursday cited sections of what it said was a draft agreement showing that Washington planned to offer to cooperate with Russia in joint military action against the two groups.