An officer and three soldiers were killed in a bomb explosion in northwestern Syria on Monday, a human rights activist said, adding that the blast was activated by suspected army defectors.

"Gunmen suspected of being army defectors blew up a bomb by remote control as an army vehicle passed by Ehssem in the countryside of the (northwestern province of Idlib), killing an officer and three soldiers, and wounding others," said Rami Abdel Rahman.

Abdel Rahman's Syrian Observatory for Human Rights had earlier reported that five soldiers were killed in clashes Monday with gunmen suspected too of being army defectors in the flashpoint central province of Homs.