Syria's peace talks hit a fresh impasse over President Bashar al-Assad Monday, as the head of Lebanon's Hezbollah vowed his Shia movement would keep fighting alongside the regime until Islamic State jihadists are defeated.

The UN's Syria envoy, Staffan de Mistura, said he had pressured Damascus to outline its approach to the crucial issue of a political transition, as negotiations in Geneva entered their second week.

De Mistura said the regime's lead negotiator Bashar al-Jaafari told him "it was... premature to talk about it. My message was (that) premature (for him) means imminent as far as we are concerned".

Assad's fate has been a key obstacle in the latest talks aimed at ending Syria's devastating five-year war, which has killed more than 270,000 people and displaced millions.

A partial ceasefire brought in last month had raised hopes for an end to the violence, which were further fuelled when Russia -- a key backer of Assad -- announced last week it would withdraw most of its troops from Syria.

But tensions have flared since, with Moscow accusing the US of "unacceptable" delays in agreeing how to punish those who break the ceasefire and warning it could resort to force against violators.

There have also been concerns about whether any peace deal could be enforced on Syria's complex battlefields, and Hezbollah vowed to stay in Syria until the Islamic State group (IS) and al-Qaeda's Syria branch are beaten.

"All that has been said about our withdrawal from Syria is false," Hezbollah's leader Hassan Nasrallah told Lebanese channel Al-Mayadeen on Monday.

Hezbollah first announced it was fighting alongside Assad's troops in 2013 and has since sent thousands of fighters to battle Syria's rebels, who are backed by its arch rival Saudi Arabia and a US-led coalition.