Kerry: Russia must stop bombing women, children in Syria

US Secretary of State John Kerry has urged Russia to implement a ceasefire in Syria, saying its bombing campaign was killing women and children in large numbers and "has to stop."

The next few days will determine if talks for a ceasefire and humanitarian access for civilians are serious, Kerry said.

UN-brokered peace talks in Geneva came to a standstill earlier this week after Syrian regime troops pressed forward on a major Russian-backed offensive around Aleppo last week. But talks over the implementation of a ceasefire continue.

"The modalities of a ceasefire itself are being discussed and the Russians have made some constructive ideas about it could be implemented," Kerry told reporters on Friday.

"But if it's just talks for the sake of talks in order to continue the bombing, nobody is going to accept that, and we will know that in the course of the next days."

If it's just talks for the sake of talks in order to continue the bombing, nobody is going to accept that

The US official said that Russia and Syrian President Bashar al-Assad have failed to comply with the UN Security Council resolution that calls for an immediate halt to the aerial bombardment of civilians and allows humanitarian access.

There is evidence that Russia is using "dumb bombs" in Syria to kill large numbers of civilians, he added.

"This has to stop. Nobody has any questions about that. But it is not going to stop just by whining about it. It's not going to stop by walking away from the table or not engaging," Kerry said, adding that the next days will "tell the story of whether or not people are serious".

The Geneva talks are the first attempt at negotiating an end to the Syrian civil war in two years.

The Syrian opposition delegation to Geneva demanded an immediate end to the Russia and Syrian bombardment and the lifting of siege imposed on rebel-held areas before taking part in any negotiations.

The US Secretary of State is set to meet with Russian, Iranian and other parties involved in the Syrian conflict for talks in Munich next week.