Syrian peace talks sponsored by the United Nations have resumed in Geneva, 10 months after they fell apart.

In a ceremony rich in symbolism, and as violent clashes continued in Syria, UN envoy Staffan de Mistura convened government and opposition envoys for a new effort to devise a political process.

"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."

He took note of the presence of diplomats from the International Syria Support Group, which unites regional and world powers and is led by the United States and Russia. But Washington has been in political flux and Mr de Mistura has said there is uncertainty about the Syria strategy of the new Trump administration.

Earlier on Thursday, Russian President Vladimir Putin - arguably the most potent international power broker in Syria's conflict - voiced hope for the success of a political settlement and said it would help defeat the "terrorist malaise".

UN-designated terrorist groups Islamic State and Fatah al-Sham, the al Qaida branch in Syria, have been excluded from the Geneva talks.

A ceasefire deal crafted by Russia, whose blistering air power has helped Syrian President Bashar Assad's forces regain key territory, and by Turkey, which supports the Western-backed rebels, has provided the backdrop to the Geneva talks. But that truce is being violated a daily basis.

Cracking through a lack of trust between the two delegations is a primary obstacle, Mr de Mistura told reporters, saying he does not expect miracles. He also acknowledged "work to be done" to unite the fragmented opposition.

Mr de Mistura said he plans to hold separate talks with the two sides on Friday, 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.

On Thursday, Nasr al Hariri, a senior member of the opposition delegation, signalled out Iran as the biggest obstacle to settling the conflict in Syria and urged Washington to stop Tehran. He complained of continued ceasefire violations.

"The guarantor countries, specifically Russia, failed to control the regime and the militias that fight with the regime. They failed to control Iran," Mr al 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 al Qaida-linked groups. Opposition media agencies also reported government air raids around the Hama countryside in central Syria.

The government has insisted that the ceasefire does not protect al Qaida-linked groups, while rebels say the agreement they signed in Ankara does. Rebels have found themselves dependent on al Qaida's battle-hardened factions since 2015 to rebuff government advances around the country.

Meanwhile, Turkish troops and Syrian opposition forces captured the centre of the Islamic State-held town of al Bab, breaking a weeks-long deadlock between the two sides at the periphery of the northern town, according to Turkey's state news agency and Syrian opposition activists.

The seizure of al Bab after a protracted fight that levelled large parts of the city brings Ankara closer to its stated goal for its months-long operation in Syria: driving IS fighters from the border and preventing Kurdish rebels in the north from linking their territories west and east of Syria along the Turkish border.

It also highlights the complexity of the Syrian terrain. Pro-government forces are just two miles south of al Bab, though clashes with the opposition forces in the area have so far been limited.

The Geneva talks are the latest bid to end Syria's catastrophic six-year war that has killed hundreds of thousands of people and displaced some 11 million others to countries like Turkey, Lebanon, and Jordan, and even into Europe.