Speaking at the end of the first day of talks in Kazakhstan with Iran, Turkey and delegations from the Syrian regime and its armed opponents, Russia's Syria negotiator Aleksandr Lavrentyev said the talks over the committee were "complex" but "moving forward."

"We are on a complex path towards forming the constitutional committee. There are some unclear issues," he told reporters.

"(We are) moving forward...I think tomorrow at the plenary session we will be able to put these issues to a general discussion," Lavrentyev said.

Delegations from Iran, Russia and Turkey arrived in Kazakhstan's capital Nur-Sultan seeking an end to the eight-year conflict in Syria while shoring up their interests in the country.

United Nations Syria envoy Geir Pedersen is also participating in the talks.

The constitutional committee is of particular interest to the UN which favours a Syrian-led resolution to the conflict.

Lavrentyev did not mention which problems specifically were stalling its formation.

One Western diplomat told AFP that Moscow will be aware of perceptions that recent rounds of the so-called "Astana process" have made little progress and may look to speed up the committee's formation.

"Even if a Constitutional Committee is created, it will then take a long time to reach a very uncertain result," the diplomat told AFP.

Any proposal would therefore be "low risk" for Moscow, whose military intervention in 2015 has helped Damascus assert control over two-thirds of the country's territory.

- Troubled Idlib -

The situation on the ground in the northwestern region of Idlib, under the administrative control of Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham (HTS), was also touched on in negotiations Thursday, Lavrentyev said.

Lavrentyev said there would be no ceasefire with HTS, but that minimising harm to "peaceful citizens" was a priority in the fight against the militant group.

Idlib has been protected from a massive regime offensive by a September deal inked by Damascus ally Russia and rebel backer Turkey.

But regime bombardment has increased since HTS took full control of the region from rival rebels in January.

Lavrentyev confirmed that prisoner swaps were another item on the agenda of talks in the Central Asian country that are also expected to address the distribution of humanitarian aid.

Russia, a backer of Syrian President Bashar al-Assad, has taken the lead role in diplomatic efforts in Kazakhstan that have largely sidelined UN diplomacy in Geneva.

Tehran, like Moscow, is an ally of Assad's regime, while Ankara has aligned itself with the rebels but has repeatedly threatened to attack Kurdish fighters on the Syrian side of its southern border that it views as "terrorists".

The capital of Kazakhstan was called Astana until last month, when it was renamed Nur-Sultan after the country's outgoing president.

It began hosting talks on Syria in January 2017.

Syria's war has killed more than 370,000 people and displaced millions since the conflict began with the repression of anti-government protests in 2011.