A Turkish military convoy was attacked en route to one of its observation posts in northwest Syria on Monday, Turkey's Defense Ministry said, adding that three civilians were killed and 12 others wounded in the airstrike. The ministry said in a statement that the attack violated Ankara's agreements and cooperation with Russia, Syrian President Bashar Al-Assad's most powerful ally. It said Russia had been told in advance about the convoy. "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," the ministry said in a statement. Turkey, which backs rebels fighting Al-Assad, blamed "regime forces" for the attack. The Britain-based Syrian Observatory for Human Rights said the attack was carried out by Syrian and Russian air forces, and was aimed at hindering the convoy's advance through Idlib province. Idlib is supposed to be protected from a massive government offensive by a Turkish-Russian deal struck in September last year. The Damascus regime denounced the convoy's crossing over from Turkey. Since late April, Syria and Russia have upped their bombardment of the Idlib region of some three million people, killing more than 860 civilians. Overnight, Syrian army troops reached the edges of Khan Sheikhoun in Idlib, a rebel official and a monitor said. The advance on the town, which has been in rebel hands since 2014, threatens to encircle and expel insurgents from their only patch of territory in neighboring Hama province. The northwest is the last major stronghold of the opposition to Assad, whose military has been waging its latest 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 border with Turkey, 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 spark a new influx. A series of truces brokered via Russian-Turkish talks have failed to end the fighting in Idlib, where Ankara has a dozen military positions. — Agencies