BEIRUT (Reuters) - At least 20 Syrian rebel fighters were killed in an air strike in the northwest province of Idlib on Monday, the UK-based Syrian Observatory for Human Rights said.

The Observatory said in a statement that a rebel commander was also probably killed in the air strike on the town of Haram. An activist named the commander as Basil Eissa, head of the Idlib Martyrs' Brigade.

Much of Idlib, which borders Turkey, has fallen to the rebels as Syrian President Bashar al-Assad's ground forces have withdrawn. But like other rebel-controlled areas, it remains vulnerable to air strikes.

(Reporting by Mariam Karouny; Editing by Janet Lawrence)