The Organisation for the Prohibition of Chemical Weapons (OPCW), an international chemical weapons watchdog, was examining an initial accounting from the Syrian regime of its chemical arsenal on Saturday, as part of a deal that headed off military action against Damascus.
The deal, spearheaded by the United States and Russia, is to be enshrined in a UN resolution, but envoys are struggling to agree on the wording and where to meet for further talks on the text.
The Syrian opposition, which has been sceptical of the US-Russian deal, rejected an offer by Iran to broker talks, saying regime ally Tehran "lacked political credibility."
Under the deal, Saturday was the tentative deadline for Syria to hand over details of its arsenal.
On the eve of the deadline, the Hague-based OPCW, which is tasked with dismantling the arsenal, said it had received "an initial disclosure from the Syrian government of its chemical weapons programme."
A UN diplomat said the OPCW had received the Syrian declaration on Thursday. "It is quite lengthy," he said.
The OPCW has postponed a meeting of its Executive Council set for Sunday that had been due to discuss how to dismantle Syria's chemical weapons programme.
The agreement, worked out as Washington threatened military action in response to a 21 August chemical weapons attack outside Damascus, requires Syria to hand over the whole of its arsenal for destruction.
It has received widespread international support, including from China, whose Foreign Minister Wang Yi said Beijing would "support the early launch of the process to destroy Syria's chemical weapons."
Wang also called for the convening of a mooted peace conference in Geneva "as soon as possible."
But the international consensus on the plan has not carried over into negotiations on the wording of a UN Security Council resolution to back it up.
The Council's five permanent members -- Britain, China, France, Russia and the United States -- have been wrangling over the text of the resolution since Monday in a bid to find common ground.
The United States, France and Britain want a strongly worded resolution, possibly under the UN Charter's Chapter VII, which could allow the use of force or sanctions to ensure compliance.
But Russia, a key ally of Damascus, opposes all references to use of force.
US Secretary of State John Kerry said he and his Russian counterpart Sergei Lavrov spoke on the telephone Friday about a "strong" UN Security Council resolution on the deal.
"We talked about the cooperation which we both agreed to continue to provide, moving not only towards the adoption of the OPCW rules and regulations, but also a resolution that is firm and strong within the United Nations," Kerry said.
"We will continue to work on that," he added.
The chemical weapons disarmament deal has done little to slow fighting on the ground, with the Syrian Observatory for Human Rights reporting that regime troops killed 15 people in a Sunni village in the central province of Hama late on Friday.
The Observatory said regime troops backed by Alawite pro-regime militias killed 12 men, two women and a child in the village of Sheikh Hadid.
Syrian President Bashar al-Assad is from the Alawite religious minority and faces an uprising that is dominated by Sunnis.
Elsewhere in the country, the Observatory said rival rebel groups carried out prisoner exchanges under the terms of a deal to end fighting over the town of Azaz, near the border with Turkey.
The truce deal between the mainstream Free Syrian Army and the Al-Qaeda-affiliated Islamic State of Iraq and the Levant (ISIS) was brokered by a third brigade, which is overseeing the ceasefire.
Tensions have spiralled between some mainstream rebel groups and ISIS in recent months, especially in northern Syria, where the opposition controls vast swathes of territory.
The Syrian opposition's National Coalition meanwhile rejected an offer from Iranian President Hassan Rouhani for Tehran to mediate talks between rebels and the regime.
"The Iranian initiative is not serious and lacks political credibility," the key opposition grouping said in a statement, pointing to Tehran's close ties to Assad's government.
French President Francois Hollande is to meet Rouhani on the sidelines of the UN General Assembly next week for talks on Syria and Iran's nuclear programme.
"What we want to see is an Iran fully engaged, like other players, in the search for a real political transition in Syria," an aide to Hollande said.