Tunisian security forces have lost track of an armed group that killed a policeman in the border near Algeria this week, a security official said on Wednesday.

"The search operation was moved to Mount Chaambi on the basis of unconfirmed information about the presence of armed men" in the area, the site of the attack, said the official from the Kasserine region.

Security forces have been hunting for the gunmen for two days near the Dernaya area but without any success, the official told AFP on condition of anonymity.

At the beginning of the operation, the security services said they were seeking five "bearded" men and that they had located their camp, where an item of Algerian origin and a Tunisian SIM card were discovered.

The five are wanted in connection with the attack on Monday in which the head of the police station in the western region of Kasserine, Anis Jlassi, was shot dead and four of his men were wounded.

Meanwhile, the interior ministry said security forces on Tuesday defused an explosive device that a man had left on a bus in Kairouan in central Tunisia. It did not provide any other details.

Clashes, strikes and attacks by hardline Islamists have multiplied across Tunisia in the run-up to the second anniversary of the start of Tunisia's revolution, which will be marked next Monday.

Members of Tunisia's militant Salafist movement, thought to number between 3,000 and 10,000, have been implicated in numerous acts of violence since last year's revolt.