Activists have reported intense clashes in north-western Syria as rebels attack a sprawling military air base.
The British-based Syrian Observatory for Human Rights said there are casualties on both sides.
Rebels have laid siege to the Abu Zuhour air base in the north-western Idlib province for months.
The Observatory and the Local Co-ordination Committees said the Syrian air force conducted several air raids during Saturday's battles to ease pressure on troops.
Rebels control much of Idlib province, which borders Turkey, although government troops still hold some areas, including the provincial capital that carries the same name.
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.