Activists say some 150 rebels and government troops have been killed in fierce fighting for control of the international airport in the northern city of Aleppo and a major military air base nearby.

The director of the Britain-based Syrian Observatory for Human Rights says that the death toll is from fighting on Wednesday and Thursday and is almost evenly divided between opposition fighters and government soldiers.

The Observatory and the Local Coordination Committees activist group say rebels and President Bashar Assad's forces are shelling each other in renewed clashes Friday in and around the airports.

Rebels launched a major attack on Aleppo's civilian airport and the nearby air base of Nairab on Wednesday. They have captured most of the "Brigade 80" force that is in charge of protecting the area as well as an army checkpoint. The airport itself and the military airfield, which have their own defences as well, both still remain in regime hands.

"The operation will continue until we control the airport and Nairab," Col. Abdul-Jabbar al-Aqidi, commander of the rebels' Military Council in Aleppo, told Al-Arabiya TV.

Control of Aleppo international airport and Nairab would be a huge strategic shift for Syria's northeastern region, giving the opposition a potential air hub enabling aid and other flights. But in order to start using the airport, the rebels first would likely have to secure all of the embattled city of Aleppo, where fighting has settled into a bloody stalemate in the streets and squares, as well as much of the surrounding countryside.

Push into Damascus

Although Assad's regime does not appear on the brink of collapse, rebels seeking his ouster have scored a string of strategic victories over the past week, seizing a large dam, a smaller air base in Aleppo province and an oil field in the east. These and other blows have shrunk the portion of the country that Assad effectively governs and could deprive his regime of resources necessary for its survival.

In recent weeks, opposition fighters have also been trying to slowly push their way into the capital, Damascus, primarily from neighbourhoods in the northeast and south sides of the city.

On Friday, the Observatory reported shelling and fighting in the Damascus suburbs of Zabadani and Daraya, where a number of soldiers were killed or wounded when their vehicle received a direct hit.

Residents in the capital said they could hear strong booms, mostly from around the northeastern neighbourhood of Jobar and nearby areas, where the fighting concentrated in the past days cutting a major highway that cuts through Damascus.

70,000 killed

The United Nations says nearly 70,000 people have been killed since Syria's crisis started in March, 2011.

In Turkey, the military on Friday again returned fire at Syria after a shell landed in a forested area inside Turkish territory, the state-run Anadolu Agency reported. There were no casualties from the round, which landed 1 kilometre from the border, across from the Syrian town of Keseb, it said.

A Turkish official could not immediately confirm the report.

The border area has seen fierce fighting in the civil war and tensions have also flared between the Turkey and the Syrian regime in the past months after shells fired from Syria landed on the Turkish side. Turkey has been firing back at Syria since five civilians were killed from Syrian shelling in October.

NATO allies Germany, Netherlands and the United States have each deployed six Patriot missile batteries near the border with Syria to protect Turkey from any spillover from the civil war in Syria.