Syrian rebels besieging a military airport near the border with Iraq were involved in serious fighting on Wednesday, September 5 opposition sources said, as rebels in the north of the country said they had shot down a fighter jet, Reuters said.

Free Syrian Army rebels have been laying siege to Hamadan airport in the city of Albu Kamal on the country's eastern frontier in Deir al-Zor province for the past three days.

Struggling to put down a 17-month-old uprising against his rule, President Bashar al-Assad has been increasingly relying on aircraft to attack the rebels, who are comparatively lightly armed with machineguns and rockets.

The opposition said the airport, where dozens of soldiers are still holding out, has been used by helicopters to launch bombing runs against rebel strongholds.

The rebels stepped up their attacks on air bases last week in an attempt to blunt Assad's air power which he has deployed to try to check rebel advances in urban and rural areas in the last month.

"The airport has effectively fallen after many soldiers defected," Nawaf al-Bashir, a senior tribal figure from Deir al-Zor who is in contact with rebels, said from Istanbul.

Abu Teif Ziad, another opposition campaigner from Deir al-Zor, said that the airport was the last base where government forces were present in Albu Kamal after rebels overran several army compounds in the town, which is situated on the Euphrates river a few kilometers from a crossing point with Iraq.

"If the Hamadan airport falls, Albu Kamal will come completely under rebel control," he said.

Attacks by rebels have already rendered two military airports in the northern province of Idlib - Taftanaz and Abu Thuhur - in operational, according to an opposition source and diplomats.