Damascus on Monday condemned the deployment of a Turkish military convoy towards a key town in northwestern Syria where regime forces are waging fierce battles with jihadists and rebels.
An AFP correspondent saw a military convoy made up of around 50 armoured vehicles including personnel carriers and at least five tanks travelling through the Idlib province.
They were headed south in the direction of the town of Khan Sheikhun, which Russia-backed regime forces entered on Sunday.
The Syrian Observatory for Human Rights reported Syrian and Russian air strikes aimed at hindering the convoy's advance through Idlib province.
The Damascus regime denounced the convoy's crossing over from rebel-backer Turkey.
"Turkish vehicles loaded with munitions... are heading towards Khan Sheikhun to help the terrorists," a foreign ministry source said, using the regime's blanket term for rebels and jihadists.
This "confirms once again the support provided by the Turkish regime to terrorist groups," state news agency SANA reported the source as saying.
The AFP correspondent saw them on their way to the town of Maaret al-Noman, which lies 15 kilometres north of Khan Sheikhun.
A Russian air strike hit the rebel vehicle leading the convoy just outside Maaret al-Noman, killing a Turkish-backed fighter from the Faylaq al-Sham group, the Britain-based Observatory said.
The AFP correspondent saw the convoy pause briefly outside the town.
After the convoy made it inside the town, Russian and Syrian warplanes targeted its outskirts in an apparent "attempt to prevent the convoy from advancing", Observatory chief Rami Abdel Rahman said.
- Failed buffer zone -
On Sunday, pro-regime forces backed by Russian air strikes took control of Khan Sheikhun's northwestern outskirts.
Any seizure of Khan Sheikhun and territory further east would encircle a patch of countryside to its south, including the town of Morek that is home to a key Turkish observation post.
Naji Mustafa, a spokesman for the National Liberation Front rebel grouping, told AFP: "The Turkish reinforcements were headed to... Morek."
Khan Sheikhun lies on the southern edge of the Idlib region, which after eight years of war is the last major stronghold of opposition to President Bashar al-Assad's regime.
The Idlib region was supposed to be protected from a massive regime offensive by a buffer zone deal signed between Russia and Turkey in September last year.
Some Turkish troops were deployed to patrol the planned buffer zone, but it was never fully implemented as jihadists refused to withdraw from the planned demilitarised cordon.
Since late April, the regime and Russia have upped their bombardment of the Idlib region of some three million people, killing more than 860 civilians.
For more than a week, deadly clashes have seen regime forces slowly advancing in the southern edges of the bastion.
Khan Sheikhun lies on a key highway connecting government-held Damascus with the northern city of Aleppo, which the regime retook from rebels in December 2016.
Jihadist group Hayat Tahrir al-Sham, led by Syria's former Al-Qaeda affiliate, controls most of Idlib province as well as parts of the neighbouring provinces of Hama, Aleppo and Latakia.