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In a ceremony rich in symbolism, and as violent clashes continued in Syria, de Mistura admitted he was not "expecting miracles" while warning of dire consequences if the talks "fail again".

"This is... our solemn responsibility...  a historical responsibility not to condemn the future generations of Syrian children to long years of bitter and bloody conflict," he said.

"The Syrian people all want an end to this conflict and you all know it," he said in a cavernous UN assembly hall, addressing the warring sides.

"They are waiting for a relief of their own suffering, and the dream of a new road out of this nightmare to a real and normal future in dignity."

This is... our solemn responsibility... a historical responsibility not to condemn the future generations of Syrian children to long years of bitter and bloody conflict - Staffan de Mistura, UN's Syria envoy

De Mistura said he plans to hold separate talks with the Syrian regime and opposition groups on Friday, adding that he is trying to devise a plan that could lead to talks over governance, a new constitution, and elections sought by the UN Security Council.

But the diplomatic initiative in the Swiss city - known as Geneva IV following three rounds that failed amid renewed fighting last year - comes at a time of new violence on the ground in Syria.

"We face an uphill task. It will not be easy. There is a lot of tension and there is a lot suffering that everyone has been bearing, but we must apply ourselves to this task," de Mistura said.

"We do know what will happen if we fail once again: More deaths, more suffering, more atrocities, more terrorism, more refugees."

They are waiting for a relief of their own suffering, and the dream of a new road out of this nightmare to a real and normal future in dignity - Staffan de Mistura, UN's Syria envoy

The Syrian opposition delegation said Iran remains the biggest obstacle to ending the conflict [TNA]

On Thursday, Nasr al-Hariri, a senior member of the opposition delegation, signalled out President Bashar al-Assad's key ally Iran as the biggest obstacle to settling the conflict in Syria and urged Washington to stop Tehran.

He complained of continued cease-fire violations.

"The guarantor countries, specifically Russia, failed to control the regime and the militias that fight with the regime. They failed to control Iran," Hariri said, referring to another key Assad backer. He said Tehran has "spread tens of thousands of sectarian fighters."

Abdulahad Astepho, a member of the opposition, said rebels would have a greater role in this round of talks, reflecting the changing dynamics inside Syria, where factions are drifting away from the exiled opposition leadership and closer to ultraconservative groups.

On Thursday, activists reported heavy clashes across the southern city of Daraa between pro-government forces and opposition factions headed by a al-Qaeda-linked group.

Opposition media agencies also reported government air raids around the Hama countryside in central Syria.

The Syrian conflict began when the Baath regime, in power since 1963 led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.