Syrian activists say fighting has intensified as government troops and rebels battle for control of the international airport in the northern city of Aleppo and nearby army bases.

The Syrian Observatory for Human Rights says clashes erupted anew on Tuesday around the airport. The rebels have been trying to seize the strategic facility for weeks, overrunning checkpoints and capturing a smaller military base that provides military protection for the airport.

But President Bashar Assad's soldiers have held on to the airport, Syria's second largest.

The Observatory, a Britain-based group that relies on a network of activists on the ground, says the rebels have also intensified their assault on the Nairab and Mannagh air bases near the Aleppo airport, as well as on an airfield nearby known as Kweires.