Jihadists from the Islamic State group seized part of the key regime air base at Deir Ezzor in eastern Syria in fierce fighting on Saturday, a monitoring group said.
"IS mounted a dawn attack on Deir Ezzor military airport and entered several areas as fierce clashes unfolded between the jihadists and regime forces there," the Syrian Observatory for Human Rights said.
The assault began with a suicide bomber targeting the main gate, followed by an artillery bombardment, the Britain-based group said.
Jihadist fighters managed to seize a missile facility in the southeast of the complex, it added.
Fighting was also reported inside Deir Ezzor city itself on Saturday.
On Friday, the Observatory said the militants were closing in on the base, nearly surrounding its beleaguered garrison.
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The IS offensive against the airport began on Wednesday, and by early Saturday had resulted in the deaths of 121 people on both sides -- 70 jihadists and 51 members of pro-government forces, it said.
Several of those killed on the government side were captured and beheaded, the Observatory said, adding that two of the dead jihadists were French nationals.
The Deir Ezzor air base is a key regime outpost from which warplanes and helicopters mount raids on jihadist positions in several areas of the war-wracked country.
IS fighters control most of Deir Ezzor province, but half of its capital remains in government hands.
The oil-rich province lies between IS-controlled Raqa province and the border with Iraq, and is a key prize for the jihadist group which declared an Islamic "caliphate" straddling the two countries in June.