(Adds details from source)

TUNIS, Nov 30 (Reuters) - Tunisian security forces have arrested two Islamist militants accused in last week's bombing in Tunis and seized a cache of weapons and explosives they were planning to use for more attacks, the interior ministry said.

In a late Sunday raid in Medenine in the south of the country, officials found Kalashnikov rifles, a suicide bomb belt, explosive materials for making car bombs, grenades and detonators, the statement said.

Tunisia is under a state of emergency since Tuesday when a suicide bomber killed 12 people aboard a bus carrying presidential guards in the capital Tunis.

Islamic State has claimed the bombing, the third major militant attack in Tunisia this year following assaults on a beach resort at Sousse and the Bardo museum in the capital, both of which targeted foreign visitors.

A Tunisian security source said the two men arrested on Sunday night were brothers. Tuesday's attacker was a street seller who lived near Ettadhamen, one of the poorer neighbourhoods of Tunis.

(Reporting by Tarek Amara; Writing by Patrick Markey; Editing by Ralph Boulton) ((pat.markey@thomsonreuters.com; +21620349006; Reuters Messaging: pat.markey.thomsonreuters.com@reuters.net))