The Syrian government reacted angrily after the convoy entered Idlib province and headed towards a key town where regime forces are waging fierce battles with jihadists and rebels.
After eight years of civil war, the jihadist-run Idlib region on the border with Turkey is the last major stronghold of opposition to President Bashar al-Assad's regime.
The region of some three million people was supposed to be protected by a Turkish-Russian buffer zone deal signed last year, but instead regime and Russian forces have upped their deadly bombardment of the bastion since late April.
Following days of inching forward on the ground, Russian-backed regime forces on Sunday took control of the edges of the town of Khan Sheikhoun in the south of the stronghold.
On Monday, an AFP correspondent saw a military convoy of around 50 armoured vehicles including personnel carriers and at least five tanks travelling southwards through the Idlib province.
The Syrian Observatory for Human Rights, a Britain-based monitor, reported Syrian and Russian air strikes aimed at hindering the convoy's advance through Idlib province.
Turkey's defence ministry "strongly" condemned the attack, which it said had killed three civilians and wounded 12.
"Despite repeated warnings we made to the authorities of the Russian Federation, the military operations by the regime forces continue in Idlib region in violation of the existing memorandums and agreements with the Russian Federation," it said in a statement.
The Damascus regime meanwhile denounced the convoy's crossing over from Turkey.
"Turkish vehicles loaded with munitions... are heading towards Khan Sheikhoun to help the terrorists," a foreign ministry source said, using the regime's blanket term for Turkish-backed rebels and jihadists.
This confirmed "the support provided by the Turkish regime to terrorist groups," state news agency SANA reported the source as saying.
Rebel killed
The AFP correspondent saw them in the town of Maaret al-Noman, which lies 15 kilometres north of Khan Sheikhoun.
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.
On Sunday, pro-regime forces backed by Russian air strikes took control of Khan Sheikhoun's northwestern outskirts.
Fighting continues to east and west of the town, the Observatory says.
Any seizure of Khan Sheikhoun 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.
It would also see the regime secure a position on a key highway connecting government-held Damascus with the northern city of Aleppo, which the regime retook from rebels in December 2016.
Naji Mustafa, a spokesman for the National Liberation Front rebel grouping, told AFP: "The Turkish reinforcements were headed to... Morek."
'Protect Khan Sheikhoun'?
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 future demilitarised cordon.
Analyst Nawar Oliver said the latest developments in Khan Sheikhoun were likely linked to a "disagreement" between both signatories.
He said Turkey sending troops was likely due to "it not being ready for the security of its troops to be threatened, or for their destiny to become at the mercy of the regime and Russia".
It could also be a "Turkish decision to protect Khan Sheikhoun", said the expert at the Turkey-based Omran Centre for Strategic Studies.
Since late April, the regime and Russia have upped their bombardment of the Idlib region, killing more than 860 civilians.
More than 400,000 people have fled their homes and dozens of health centres have been targeted, the United Nations said.
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.
Syria's war has killed more than 370,000 people and displaced millions since it started with the brutal repression of anti-regime protests in 2011.
