TUNIS (REUTERS) - Tunisia said on Thursday it had arrested four militants and seized arms and explosives in the southern town of Sidi Bouzid following increased security checks prompted by fears of attacks by radical Islamists during New Year celebrations.

The detentions followed the arrest of seven Salafists, or ultra-conservative Islamists, on Sunday in the western town of Kasserine.

"The national police arrested four terrorists belonging to a banned organization on Wednesday night. A Kalashnikov piece, munitions and explosives were seized during the operation," Tunisia's interior ministry said in a statement.

Local media reports said police had exchanged gunfire with the militants in Sidi Bouzid on Wednesday night and that a police officer has been injured. The ministry declined to confirm the reports.

State media said on Monday U.S. and Libyan forces had captured the leader of Tunisia's Islamist militants, Ansar al-Shariaa, in the Libyan city of Misrata, but his group denied he had been taken and the U.S army denied any involvement.

The interior ministry's statement did not say whether the four militants arrested belonged to al-Sahriaa's organisation.

Last week, Tunisian police seized 60kg of gold, worth around US$2.3 million (S$ 3 million), as they checked vehicles in the same city of Sidi Bouzid. The authorities said it was too early to say whether the gold had come from or was destined for militant groups.

The seven Salafists arrested on Sunday had been distributing leaflets demanding a ban on New Year celebrations and had threatened bakeries selling cakes for the occasion, local media said.

New Year celebrations used to be a big event in Tunisia, the most secular country in the Arab world, but they have faded since the revolution that toppled President Zine el Abidine Ben Ali in early 2011, paving the way for the moderate Islamist Ennahda party to take power.

After a recent political crisis over the role of Islam in politics, the Ennahda party agreed to resign and will hand over to a caretaker government by January 14, the third anniversary of Ben Ali's fall.

Ultra-conservative Salafists, who want to impose an Islamic state, returned from exile or prison after Ben Ali was toppled.