Three people were killed and 12 were wounded in airstrikes on Turkish military convoy carrying ammunition after it crossed into northern Syria on Monday, bound for a rebel-held stronghold, Turkish Defense Ministry said.
The strikes hit near the highway where the convoy was moving, the activists added.
Ankara condemned the attack that "violates agreements and cooperation with Russia," Turkey's Anadolu state news agency said.
It was not immediately clear whether it was Syrian government or Russian warplanes that struck near the convoy but the development marked a sharp escalation in tensions in the northwestern province of Idlib where Syrian troops have been on the offensive for weeks.
>> Read more: As U.S. and Turkey argue over Idlib, Syria's wild south reawakens | Analysis
Syrian state media SANA said the convoy entered Syria on Monday to help insurgents in the town of Khan Sheikhoun who are fighting a government advance, calling it an act of aggression.
The advance not only threatens the town, in rebel hands since 2014, but also to encircle insurgent fighters in their only patch of territory in neighboring Hama province.
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Ankara backs some of the rebels in the northwest and has deployed forces into the Idlib region under deals with Russia, President Bashar Assad's most powerful ally.
Syria's Foreign Ministry slammed Turkey, saying the convoy consisted of armored vehicles loaded with ammunition and was heading to Khan Sheikhoun, a major rebel-held town in Idlib province, the country's last rebel stronghold.
Turkey's private DHA news agency said Syrian government planes targeted the route of a Turkish military convoy carrying reinforcement vehicles and personnel. It said the convoy was heading toward two Turkish observation posts in the region when it came under Syrian aircraft fire.
The report said the convoy could not proceed because the route between Maaret al Numan and Khan Sheikhoun was targeted, and sheltered at a safe location. The agency did not report any casualties.
The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said airstrikes believed to be Russian struck near the highway and forced the nearly 25-vehicle Turkish convoy to stop.
Mazen al-Shami, an opposition activist based in Idlib, also said that warplanes struck areas near where the Turkish convoy was moving. The Sham Network, an activist collective, posted photos of the Turkish convoy — a mushroom of smoke, apparently from the airstrike, could be seen in the distance.
The northwest is the last major stronghold of the opposition to Assad, whose military has been waging its latest offensive there since late April with Russian help.
The surge in violence since late April has killed at least 500 civilians and uprooted hundreds of thousands, many toward the Turkish border, the United Nations says.
A witness said a Turkish military convoy, with rebel allies, entered Idlib on Monday but was stopped because of heavy bombing there.
Citing a Foreign Ministry source, SANA said the convoy loaded with munitions would not affect "the determination of the Syrian Arab Army to keep hunting the remnants of terrorists" in Khan Sheikhoun or elsewhere.
Colonel Mustafa Bakour of the Jaish al-Izza rebel faction said battles raged on the outskirts of the town. Fighters arrived to reinforce the frontline, he said, including some from the National Army, a Turkey-backed rebel force based further north near the border.
Khan Sheikhoun, along a main highway stretching from the capital Damascus to Aleppo city, was bombed with sarin in 2017, an attack that killed dozens on people, wounded hundreds others and prompted a U.S. missile strike.
An investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons blamed the attack on the Syrian government. Damascus denies using such weapons.
Since a brief ceasefire collapsed this month, the Syrian army has closed in on Khan Sheikhoun from the east and west.
Pro-government forces arrived at the northwest flank of the town under heavy airstrikes and were fighting to march into it, the Syrian Observatory for Human Rights said.
Residents said warplanes pounded the town and nearby positions during the night.
Jack Khoury contributed to this Report.