A Turkish military convoy crossed into jihadist-run northwest Syria on Monday, its path blocked by advancing regime troops as tensions soared between Damascus and Ankara.
Rebel-backer Turkey said its forces were targeted by an air strike, while the Syrian regime accused Turkish forces of backing "terrorists".
The convoy had entered Idlib province before heading towards a key town where Russian-backed regime forces are waging a fierce battle to retake the area from jihadists and rebels.
Turkey claimed an air strike hit its convoy, killing three civilians, though a war monitor said a Russian air raid took the lives of three rebels in the surrounding area.
Russian President Vladimir Putin said Monday that Moscow supports the Syrian army's offensive against "terrorists" in the northern province of Idlib, during a press conference with French President Emmanuel Macron.
"We support the efforts of the Syrian army... to end these terrorist threats" in Idlib, Putin said after Macron urged respect for a ceasefire in Idlib.
After eight years of civil war, the jihadist-run 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 regime and Russian forces have upped their deadly bombardment there since late April.
After days of inching forward, Russian-backed regime ground forces on Sunday entered the key town of Khan Sheikhun in the south of the stronghold.
On Monday afternoon, a new loyalist advance saw pro-Damascus fighters take control part of the highway north of the town, effectively blocking the Turkish military convoy from continuing south.
The Syrian Observatory for Human Rights, a Britain-based monitor with a network of contacts in Syria, said this would stop the convoy ever reaching a Turkish monitoring post south of Khan Sheikhun.
Late Monday, the Observatory said the convoy was at a standstill in the village of Maar Hattat on the highway just north of Khan Sheikhun, where an AFP correspondent had seen it in the afternoon.
Russian and regime air strikes as well as heavy loyalist artillery fire hit surrounding areas, it said.
Regime forces were in control of more than half of Khan Sheikhun late Monday, with remaining anti-Assad fighters surrounded on three sides in its west with no way out except towards the south, the Observatory said.
- Deadly air strike -
Analysts say regime forces want to retake the key road that connects Damascus with the northern city of Aleppo, both of which they control.
Earlier, an AFP correspondent saw a military convoy of around 50 armoured vehicles including personnel carriers and at least five tanks travelling southwards along the highway.
The Observatory reported Syrian and Russian air strikes aimed at hindering the convoy's advance.
Turkey's defence ministry "strongly" condemned the attack, saying regime operations were "in violation of the existing memorandums and agreements with the Russian Federation".
The Damascus regime meanwhile denounced the convoy's crossing from 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 confirmed "the support provided by the Turkish regime to terrorist groups," state news agency SANA reported the source as saying.
On Monday morning, a Russian air strike hit the rebel vehicle leading the convoy just outside Maaret al-Noman, 15 kilometres (nine miles) north of Khan Sheikhun, the Observatory said.
The strike killed a Turkish-backed fighter from the Faylaq al-Sham group and two other opposition fighters, it said.
After the convoy entered the town, Russian and Syrian warplanes targeted the area in an apparent "attempt to prevent the convoy from advancing", Observatory chief Rami Abdel Rahman said.
- 'Protect Khan Sheikhun'? -
On Sunday, pro-regime forces backed by Russian air strikes took control of Khan Sheikhun's northwestern outskirts.
Fighting continues to the east and west of the town, according to the Observatory.
The seizure of Khan Sheikhun and territory further east would encircle a patch of countryside to its south, including the town of Morek where a Turkish observation post is situated.
The Turkish army earlier said the convoy was heading towards Morek.
Analyst Nawar Oliver said Turkey had likely sent the convoy to avoid its troops being "threatened" or placed "at the mercy of the regime and Russia".
It may have also taken a "decision to protect Khan Sheikhun", he said.
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, the United Nations says.
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 in 2011.