Islamic State fighters killed at least 14 Syrian soldiers in a fierce attack on a military airport northeast of Damascus, the Syrian Observatory for Human Rights monitoring group reported.
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The British-based Observatory said the ultra-hardline group had launched the assault the day before near the Dumeir airport some 40 km (25 miles) from the capital, taking over several positions in the area held by the Syrian army.
ISIS controls vast areas of eastern Syria including most of Deir al-Zor province and its de facto Syrian capital of Raqqa.
It seized the historic city of Palmyra, some 200 km from Damascus, in December for the second time in the six-year Syrian conflict, driving out government forces which with Russian military support had recaptured it in March.
The Islamic State, which declared its self-styled caliphate across areas of northern Iraq and eastern Syria as it swept through the region in 2014, is being beaten in its Iraqi stronghold of Mosul.
But it has been on the offensive in recent weeks in Syria, also waging attacks against long-besieged pockets of government control in the eastern town of Deir al-Zor.