(Clarifies in paragraph 1 that Turkey did not specify who was responsible for air strike)
By Ellen Francis and Tuvan Gumrukcu
BEIRUT/ANKARA, Aug 19 (Reuters) - Turkey said an air strike on a Turkish military convoy in northwest Syria killed three civilians and wounded 12 on Monday as Syrian troops advanced against rebels inside a key town.
Air strikes hit near the convoy that was approaching front lines in Idlib province, two Turkey-backed rebel sources and a war monitor said.
Ankara, which backs some of the rebel factions in the northwest, has forces stationed in Idlib under deals with Russia, Syrian President Bashar al-Assad's most powerful ally.
Syrian state media said the despatch of the convoy into Syria was an act of aggression and it had entered to help insurgents fighting an army advance in Khan Sheikhoun, a town in rebel hands since 2014 that was bombed with sarin gas in 2017.
Syrian government forces and their allies entered the town on Monday night, seizing part of it as ground battles raged inside, the monitor and one of the rebel sources said.
The advance threatens to encircle insurgents in their only patch of territory in nearby Hama province and could put a Turkish observation post there at risk.
A series of truces brokered via Russian-Turkish talks has failed to end the fighting in Idlib, where Ankara has a dozen military positions.
The northwest is the last big stronghold of the opposition to Assad, whose military has been waging an offensive there since the end of April with Russian help.
The escalation has killed at least 500 civilians and uprooted hundreds of thousands, many stranded near the Turkish border, the United Nations says.
Turkey, which hosts some 3.6 million Syrian refugees and warns it can not accept more, fears the onslaught in Idlib could cause a new influx.
Turkey's Defence Ministry said the military convoy was attacked en route to one of its observation posts in the northwest. It said the attack violated Ankara's agreements with Moscow, which had been informed in advance about the convoy.
HEAVY AIR STRIKES
The Syrian Observatory for Human Rights, the British-based monitoring group, said the convoy was stuck on a highway after air strikes prevented it from continuing southwards. It said a fighter accompanying the convoy was killed.
It said the army had seized a hill on the highway at the edge of Khan Sheikhoun earlier under the cover of heavy air strikes and shelling, cutting the main route in and out, The army has closed in on the town from the east and west since a brief ceasefire collapsed this month.
Colonel Mustafa Bakour of the Jaish al-Izza rebel faction said fighters had arrived in the morning to reinforce the front line, including some from the National Army, a Turkey-backed rebel force based further north near the border.
Residents said warplanes pounded the town as well as nearby towns and villages in the south of Idlib.
France called on Friday for an immediate end to the fighting and condemned air strikes on camps for the displaced.
In a statement that did not specifically mention strikes against a Turkish military convoy, the United States said it was closely watching Syrian government military operations in northwest Syria. The Pentagon called on all parties, including Russia and Syria, to avoid large-scale military offensives and de-escalate violence in the area.
The sarin attack on Khan Sheikhoun in April 2017 killed dozens of people and prompted a U.S. missile strike. The town is located along a highway stretching from the capital, Damascus, to Aleppo city.
An investigation by the United Nations and the Organisation for the Prohibition of Chemical Weapons blamed the sarin attack on the Syrian government. Damascus denies using such weapons. (Reporting by Ellen Francis and Tom Perry in Beirut and Khalil Ashawi, Sarah Dadouch, Tuvan Gumrucku, and Dominic Evans in Turkey; Additional reporting by Idrees Ali in Washington; Editing by Angus MacSwan, Mark Heinrich and Peter Cooney)