Russia and Turkey, the two countries responsible for a rickety agreement to keep the peace along Syria's last major frontline, gave sharply conflicting accounts on Thursday of an attack on a Turkish military outpost.
Turkey said one of its observation posts in Idlib province had come under deliberate attack from Russian-backed Syrian government forces. Three Turkish soldiers were wounded from 35 mortar shells fired on the post.
But Moscow said the attack was carried out not by President Bashar al-Assad's government but by his rebel enemies, and that Turkey had asked Russia to launch air strikes in response, supplying coordinates of rebel targets. The Russian defence ministry said it carried out four successful strikes.
Idlib is the last remaining bastion for anti-government rebels after eight years of civil war.
Russia, which supports Assad, and Turkey, long a backer of the rebels, co-sponsored a de-escalation agreement for the area that has been in place since last year but faltered in recent months, forcing hundreds of thousands of civilians to flee.
Russia said late on Wednesday that a full ceasefire had been put in place in the area. But Turkey denied this, saying it still reserved the right to strike back if necessary.
"Right now, it is not possible to say a full ceasefire is in place but our efforts on this with Russia are continuing," Foreign Minister Mevlut Cavusoglu said at a news conference with his French counterpart.
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"If these (attacks) continue, we would do what is necessary, nobody should doubt that," he said. "Russia as the guarantor country should put pressure and we expect them to do that."
The apparent factual dispute between the two main outside powers in northern Syria highlights the erosion of the de-escalation deal, which was agreed last year to shield the area from a full-on government assault.
The rebel-held pocket is home to hundreds of thousands of people who fled other parts of Syria as government forces advanced through the country since Moscow joined the war on the side of Assad in 2015, tipping the conflict in his favour.
Since April this year, government forces have increased their shelling and bombing of the area, killing scores of people and sending hundreds of thousands again to flight.
The rebels say the government action is part of a campaign for an assault that would breach the de-escalation agreement. The government and its Russian allies say the action is in response to rebel violations including the presence in a demilitarised zone of fighters blacklisted as terrorists.
Turkey has troops stationed at several locations in the area to monitor the agreement.
The Syrian Observatory for Human Rights, a war monitor, said air strikes on rebel-held areas had paused overnight, but resumed on Thursday morning. They targeted the area around Khan Sheikhoun, an opposition-held town in southern Idlib province, it said, adding that the insurgents had also fired artillery at government forces.
Turkey has pushed Russia to rein in the government's offensive while Russia has said Turkey must curb the jihadist groups that dominate in Idlib.