Rebels have stormed parts of Taftanaz military airport in Syria's northwest Idlib province, but state media reported on Thursday that the fighters had been repelled by troops.
A rebel speaking from near the Taftanaz base overnight said the base's main sections were still in loyalist hands but fighters had managed to infiltrate and destroy a helicopter and a warplane on the ground.
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Rami Abdulrahman, head of the opposition-aligned Syrian Observatory for Human Rights which monitors the conflict from Britain, said as many as 800 fighters were involved in the assault, including Islamists from Jabhat al-Nusra, a powerful group that Washington considers a terrorist organisation.
Taftanaz is mainly a helicopter base, used for missions to resupply army positions in the north, many of which are cut off by road because of rebel gains, as well as for dropping crude "barrel bombs" of explosives on rebel-controlled areas.
Rebels have been besieging air bases across the north in recent weeks, in the hope it will reduce the government's power to carry out air strikes and resupply regime-held areas.
Near Minakh, another northern air base that rebels have surrounded, government forces have retaliated by regularly
shelling and bombing nearby towns.
Meanwhile, opposition activists said warplanes struck a residential building in another rebel-held northern town, Hayyan, killing at least eight civilians.
Video footage showed men carrying dismembered bodies of children and dozens of people searching for victims in the rubble of the destroyed building, shouting: "God is greatest."
The provenance of the video could not be independently confirmed.
In addition to their tenuous grip on the north, the rebels also hold a crescent of suburbs on the edge of Damascus, which have come under bombardment by government forces that control the centre of the capital.
On Wednesday, according to opposition activists, dozens of people were incinerated in an inferno caused by an airstrike on a petrol station in a Damascus suburb where residents were lining up for precious fuel.
The civil war in Syria has become the longest and bloodiest of the conflicts that rose out of uprisings across the Arab
world in the past two years.
More than 60,000 people have been killed in the 21-month-old uprising and civil war, the UN said this week, sharply raising the death toll estimate in a conflict that shows no sign of ending.