"Troops and pro-regime militia stopped the attack that IS launched on the Deir Ezzor military airport," the Syrian Observatory for Human Rights said, adding that both sides suffered heavy losses in the fighting.
The militants had withdrawn to the edges of the base, a day after managing to seize a southeastern part of the complex, it said.
The Observatory said more than 100 militants had been killed in fighting for the base since Wednesday, when they launched an operation to try to take the airport.
Pro-regime forces also suffered heavy casualties, with some 59 troops killed, it said.
State news agency SANA said Syrian army units had "repelled an attempt" by IS fighters to attack positions at the base, but provided no further details.
The Deir Ezzor air base is a key regime outpost from which warplanes and helicopters mount raids on militant 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 militant group which declared an Islamic "caliphate" straddling the two countries in June.
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The Arab spring was a big joke . Does any one know who's fighting whom in that region ?