"Terrorists" in Tunisia have abducted and decapitated an off-duty policeman, media reports on Monday cited the interior ministry as saying.

The officer and his brother were in a car in the Kef area of the northwest when gunmen seeking to rob them immobilized the vehicle.

During the robbery, the attackers discovered that the man was in the police and they kidnapped and later killed him, radio stations Mosaique and Express-FM quoted a ministry spokesman as saying.

The man's brother was not harmed in the attack which happened on Thursday but has only just been reported.

Security forces are now engaged in a mopping up operation in the mountainous area, which is close to the frontier with Algeria.

Since the revolution of January 2011, Tunisia has seen a rise in Islamist extremism. For the past two years, the army has been hunting jihadists in the border area.

Deadly ambushes by gunmen on the police and army are common in the region, where cross-border smuggling of fuel and food is also rife.