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.
"Turkish vehicles loaded with munitions... are heading towards Khan Sheikhun to help the terrorists... which confirms once again the support provided by the Turkish regime to terrorist groups," a foreign ministry source quoted by the state news agency SANA said.
The Syrian Observatory for Human Rights, a Britain-based war monitor, reported Syrian and Russian air strikes aimed at hindering the convoy's advance through Idlib province.
The Idlib region is supposed to be protected from a massive regime offensive by a buffer zone deal signed between government ally Russia and rebel backer Turkey in September.
But it was never fully implemented, as jihadists refused to withdraw from the planned demilitarised zone.
On Sunday pro-regime forces entered Khan Sheikhun in Idlib province for the first time since they lost control of it in 2014.
They are battling to seize a key highway connecting government-held Damascus with the northern city of Aleppo, which the regime retook from rebels in December 2016.
Dozens from both sides have been killed in the latest fighting.
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.