Syrian rebels attacked a sprawling military air base in the country's northwest on Saturday, April 27, while in the south opposition forces assaulted a string of army checkpoints and positions, activists said, according to The Associated Press.

The raids follow nearly two weeks of advances by Syrian troops, mostly in the suburbs of the capital, Damascus, and areas near the Lebanese border in the central province of Homs.

In Saturday's fighting at the Abu Zuhour air base in northwestern Idlib province, the Britain-based Syrian Observatory for Human Rights said there were casualties on both sides. The base has been under rebel siege for months.

The Observatory and the Local Coordination Committees said the Syrian air force conducted several air raids during the fighting to ease pressure on government troops inside the air base.

State-run news agency SANA quoted a military official as saying the troops repelled the attack and inflicted "great losses" on the attackers.

Rebels control much of Idlib province, which borders Turkey, although government forces still hold some areas, including the provincial capital that carries the same name.

Elsewhere, the Aleppo Media Center said rebels had entered the Kweiras military air base in Aleppo province and destroyed its operations room. The base has also been under siege for months.

In the southern province of Daraa, also known as the Houran plains, the Observatory and the LCC said rebels had launched a new offensive called "the Houran Volcano" in which they are targeting army checkpoints and positions.

The Observatory said there were losses on both sides but had no immediate breakdown.