The Syrian government said on Monday it was waiting before declaring void a ceasefire over the rebel-held Idlib province, hours after radical militants missed a deadline to leave a buffer zone.
A Russian-Turkish truce reached last month staved off a government military offensive to retake the north-western province, giving “radical fighters” until October 15 to vacate a proposed demilitarised zone.
Syrian Foreign Minister Walid Muallem said his government was waiting on its ally Russia’s reaction on whether the agreement had been implemented, after the militants failed to withdraw overnight.
"We must give the time ... to judge whether the agreement was fulfilled or not," Mr Muallem said at a news conference in Damascus.
"We have to wait for the Russian reaction. Russia is monitoring and following the situation."
The minister expressed hope that Turkish President Recep Tayyip Erdogan would still be able to "fulfil the agreement from his part".
But he hinted that a full-scale military offensive was possible, saying militants "must be removed" from their last stronghold in Idlib.
"We have to wait, but at the same time, our troops are still ready around Idlib," Mr Muallem said.
A senior Syrian opposition leader based in Turkey said the deal remained viable.
Ahmed Tumah, who heads an opposition government backed by Turkey, says rebels had lived up to their part of the deal by withdrawing their heavy weapons from the demilitarised zone.
Mr Tumah insisted the deal did not have a timeline and would go on "until we find a successful political solution".
He said he hoped it would translate into a complete ceasefire.
But hardline militant groups, including Hayat Tahrir Al Sham, control more than two-thirds of the planned buffer area, and more than half of the rest of Idlib. And the Al Qaeda-affiliated group vowed to continue fighting.
"We have not abandoned our choice of jihad and fighting towards implementing our blessed revolution," the group said hours before the withdrawal deadline.
This poses a “significant challenge” to the future of the deal, said Elizabeth Tsurkov, a research fellow specialising in Syria at the Forum for Regional Thinking. “Turkey will need to back up its diplomatic pressure on the jihadists to abide by the deal,” she said.
The future of the deal is “clearly very shaky”, said Joshua Landis, a Syria expert and head of the Center for Middle East Studies at the University of Oklahoma.
Part of why militants were reluctant to withdraw is that their leadership expected to be killed, he said. “That’s why senior fighters are so terrified as they are well known to the security apparatus. They cannot easily slink away through the border.”
He added: “The whole thing is murky mess. The next step is that Turkey and Russian troops police this buffer zone.”
This meant that more fighting was likely. “Assad and Russia are quite clear they intent to take back Idlib province.”
The deal was already on shaky ground ahead of the deadline expiration. On Saturday, rebels fired mortars from the buffer zone, days after they were supposed to have withdrawn such weapons from the area.
The removal of heavy weapons from a 15 to 20-kilometre-wide horseshoe-shaped buffer zone around Idlib was designed to prevent Latakia, Hama, and Aleppo provinces from being targeted by rebel mortar, artillery and rocket fire.
Unlike the radical militants, rebel groups are not required to leave the buffer zone.
The National Liberation Front, a coalition of rebel groups backed by Turkey, said its fighters would remain in place armed with "light and medium weapons".
"The agreement is only about heavy weapons," NLF spokesman Naji Abu Huzaifah said.
"Our forces will stay and be ready in case the Russians and regime break the agreement."
Overall the deal struck between Russian President Vladimir Putin and Turkish President Erdogan in Sochi on September 17 was designed to prevent an all-out military assault on Idlib, which after seven years of civil war is the last rebel-held province and is believed to be home to up to 3 million civilians.
Also beyond Damascus's control is north-east Syria, which is controlled by a Kurdish faction and its local allies. While not directly fighting the government, the Syrian Democratic Forces (SDF) control about 30 per cent of the country, and have established a system of local government based on the philosophy of jailed leader of the outlawed Kurdistan Workers’ Party Abdullah Ocalan.
In recent months, Syria’s Kurds have pursued talks with Damascus aimed at obtaining greater local autonomy.
But this proposal was rejected by the foreign minister on Monday, who said: “We do not accept federalism.”
He added: “After Idlib, our goal will be east of the Euphrates”, referring to the area controlled by the SDF, which is backed by the US.
"The Syrian state's decision is to take back control. They can choose if it will be through dialogue or other means."