Syrian rebels seeking to topple President Bashar Assad have fought intense battles with his troops at two military air bases in northern Syria, activists said.
The fighting raged inside the sprawling Abu Zuhour air base in north-western Idlib province and the Kweiras military air base in northern Aleppo province.
The British-based Syrian Observatory for Human Rights said at least seven fighters were killed in the fighting in Abu Zuhour, in addition to an unknown number of soldiers. The group said the Syrian air force conducted an airstrike on Abu Zuhour village during the fighting to ease pressure on government troops inside the air base.
Rebels control much of Idlib and Aleppo provinces, which border Turkey, although government troops still hold some areas including the provincial capital of Idlib province and parts of the city of Aleppo, Syria's largest urban centre.
The fighters entered the two air bases on Saturday. Both have been under siege for months.
Syria's conflict started with largely peaceful anti-government protests in March 2011 but eventually turned into a civil war. More than 70,000 people have been killed, according to the United Nations.
US President Barack Obama's administration said on Thursday that intelligence indicates that government forces likely used chemical agents against rebels in two attacks.
Washington's declaration was its strongest on the topic so far, although the administration said it was still working to pin down definitive proof of the use of chemical weapons. It held back from saying Damascus had crossed what President Obama has said would be a "red line" prompting tougher action in Syria.
Both sides of the civil war accuse each other of using the chemical weapons. The deadliest such alleged attack was in the Khan al-Assal village in the Aleppo province in March. The Syrian government called for the UN to investigate alleged chemical weapons use by rebels in the attack that killed 31 people.
Syria, however, has not allowed a team of experts into the country because it wants the investigation limited to the single Khan al-Assal incident, while UN Secretary-General Ban Ki-moon has urged "immediate and unfettered access" for an expanded investigation.