Five Turkish soldiers have been killed and five others wounded by Syrian army artillery fire in north-western Syria, Turkey’s defence ministry says.
Shells reportedly struck a Turkish observation post at Taftanaz airbase in opposition-held Idlib province, where pro-government forces are mounting a major offensive.
Turkish forces retaliated by shelling Syrian army targets in the region.
A week ago, eight Turkish military personnel died in a similar attack.
Syrian soldiers, supported by Iran-backed militiamen and Russian air strikes, have captured dozens of towns and villages in Idlib since December.
Almost 600,000 civilians, the vast majority of them women and children, have been displaced by the fighting, according to the United Nations.
Turkey, which backs the Syrian opposition, has given the Syrian army until the end of February to pull back behind a line of 12 Turkish observation posts set up in Idlib and surrounding provinces to monitor a ceasefire that has now collapsed.
It has also sent hundreds of tanks and thousands of soldiers to reinforce the posts.
A Turkish defence ministry statement said the five soldiers were killed on Monday in an “intense assault that targeted our elements sent as reinforcement to the region with an aim to prevent clashes in Idlib, ensure our border security and stop migration and human tragedy”.
“Targets identified in the region were immediately targeted intensively… The targets were destroyed and the blood of our martyrs was not left on the ground,” it added.
Turkish media and the Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, reported that Syrian pro-government forces shelled Taftanaz airbase, about 13km (8 miles) north-east of the city of Idlib.
The SOHR said six Turkish soldiers and four Syrian rebels were killed in the attack.
Videos posted online purportedly showed helicopters transferring the wounded soldiers across the border to hospitals in Turkey.
Turkish government spokesman Fahrettin Altun wrote on Twitter: “Turkey retaliated against the attack to destroy all enemy targets and avenging our fallen troops.”
“The war criminal, who ordered today’s heinous attack, targeted the entire international community, not just Turkey,” he added.
There was no immediate comment from the Syrian army or government.
Last Monday, seven Turkish soldiers and a civilian contractor died when shells hit a position in Idlib, Turkey’s defence ministry said. In response, Turkish forces “neutralised” 76 Syrian soldiers, it added.
Syrian state media said there were no casualties. But the SOHR reported that at least 13 soldiers were killed by Turkish fire in Idlib, Latakia and Hama provinces.
Two days later, Turkish President Recep Tayyip Erdogan threatened to drive back the Syrian troops in Idlib unless they withdrew behind the 12 observation posts set up by Turkey as part of a 2017 de-escalation agreement with Russia and Iran.
“We hope that the process of the regime pulling back behind our observation posts is completed in the month of February,” he said. “If the regime does not pull back during this time, Turkey will have to do this job itself.”
However, the Syrian army vowed to continue its assault and subsequently captured the major town of Saraqeb, which straddles the intersection of the strategic M4 and M5 highways.
The Syrian armed forces command has called the presence of Turkish forces on Syrian territory an “illegal and a flagrant act of aggression”.