Syrian troops and rebels have fought intense battles around a strategic air base in the country's north and a suburb of the capital that government forces have been trying to capture since last month, reports say.
The fighting is part of the escalating violence in a conflict that the United Nations estimates has killed more than 60,000 people since the revolt against President Bashar Assad began in March 2011.
The Britain-based Syrian Observatory for Human Rights said rebels stormed parts of the Taftanaz air base in the north-western province of Idlib before withdrawing.
The state-run SANA news agency said government forces protecting the base "repelled the terrorists' attempt to attack the airport" and inflicted heavy losses.
The Observatory said rebels resumed their assault early today. They have been pursuing a strategy of attacking airports and military airfields, targeting five air bases in Idlib and the nearby province of Aleppo, trying to chip away at the government's air power.
With its troops struggling to make headway against the rebels in the field, the government has increasingly relied on its warplanes and helicopters to target opposition forces.
The Observatory and the Local Co-ordination Committees, another activists group, reported clashes, air raids and shelling in several suburbs of the capital Damascus, including Daraya, which the regime has been trying to capture from hundreds of opposition fighters for weeks.
The pro-government al-Watan daily said the army had destroyed rebel strongholds in Daraya and inflicted heavy losses, adding that the area would be declared safe later in the day.
Daraya lies in a strategic location, and a government takeover there would provide a boost to the regime's defence of Damascus.
Al-Watan said thousands of rebel fighters from the extremists Jabhat al-Nusra group have holed up in Daraya in preparation to storm Damascus. Jabhat al-Nusra, which has been branded a terrorist organisation by the US and which Washington claims is affiliated with al Qaida, has been among the most effective fighting forces on the rebel side in the battle to oust Mr Assad.