Geneva (AFP) - Syria's peace talks hit a fresh impasse over President Bashar al-Assad, as the head of Lebanon's Hezbollah vowed his Shiite 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.
"We went to Syria to help keep the country from falling into the hands of Daesh (IS) and Al-Nusra Front... So long as we have a responsibility to be there, we will be there."
- Assad hampers talks -
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.
Its support has been crucial for keeping the regime in power, but the opposition has insisted the president's departure must be part of any peace deal agreed at the talks.
Member of the High Negotiations Committee (HNC) opposition umbrella group, Yahya Kodmani, on Sunday accused the regime of being "obstinate".
"We hope that Russia will use its powers to pressure the Assad regime in order to move into serious negotiations," he said.
But Jaafari rejected any suggestion that Assad would step down from any new government, saying his future and the political transition were "two separate issues".
"President Assad has nothing to do with the... talks," he said, insisting that the subject of the president "is something that is already excluded from the scene".
He also insisted Damascus was committed to the peace process, and that his delegation had "clear instructions from our leadership to engage seriously in these talks".
Conceding that progress remained slow, de Mistura stressed it was vital that opposing sides reach a basic understanding on how to move to a second round of talks, tentatively scheduled for next month.
- Russia warns on ceasefire -
He highlighted the positive impact of the negotiations, which he said have helped to maintain a fragile ceasefire declared on February 27.
But tensions flared between Russia and the US over the truce, with Russian Lieutenant General Sergei Rudskoy accusing Washington of showing "no readiness" to agree how to enforce it and warning Russia would resort to force itself if necessary.
"The delay in the entry into force of the rules agreed upon for responding to violations of the ceasefire in Syria is unacceptable," he said in a statement.
Moscow would only use force "after receiving credible evidence of armed groups' systematic violations" of the truce, he added.
The truce has broadly held since being declared last month and allowed life-saving aid to reach tens of thousands of Syrians stranded in besieged areas.
It does not include IS and the Al-Qaeda linked Al-Nusra Front, however, who continue to be targeted in Russian air strikes and government offensives.
At least 26 pro-government fighters were killed battling Islamic State near Palmyra on Monday as Damascus stepped up a bid to recapture the ancient city, the Syrian Observatory for Human Rights said.