A Tunisian policeman has been beheaded by Islamist militants in an attack near the Algerian border, officials say.

An interior ministry spokesman said the officer and his brother had been seized from their car in the Kef area of north-west Tunisia.

The brother was later released but the officer was killed by his abductors.

The attack came as Tunisia prepares for a presidential election run-off this month in the latest stage of its transition to democracy.

The leader of the secular leaning Nidaa Tounes party, Beji Caid Essebsi, will face the incumbent interim president, human rights campaigner Moncef Marzouki, in the poll.

The winner is expected to be Tunisia's first freely elected leader.

Analysts say one of the new president's challenges will be to tackle Islamist extremism, which has grown since the ousting in 2011 of long-time authoritarian leader, Zine El-Abedine Ben Ali.

Bus ambush

No group has claimed responsibility for the beheading.

Last month, five members of the security forces were killed in an ambush by militants on a bus, also in the Kef area.

Security personnel were searching for those behind the latest attack, the interior ministry said.

Some observers say Tunisia has fared better than other countries that witnessed an Arab Spring uprising.

However, last year Islamist militants were blamed for killing two prominent leftist politicians, which created a political crisis.

And there are claims that Tunisia's radical Islamists have been becoming more active elsewhere in the region.

According to the Soufan Group, a strategic consultancy, there are more Tunisians among foreign jihadists in Syria and Iraq than nationals from any other single country.