Islamic State (IS) militants on Saturday claimed responsibility for an attack in a Tunisian seaside resort that killed 39 people.In a statement released on Twitter, the group identified the gunman as Abu Yahya al-Qayrawani, describing him as a "soldier of the caliphate" and most of the killed as "subjects of states that make up the crusader alliance fighting the state of the caliphate".On Friday, the gunman opened fire on the popular beach resort in Sousse city, south of the capital, making it the deadliest attack in the country's recent history.Following the attack, Prime Minister Habib Essid announced that Tunisia plans within a week to close down 80 mosques that remain outside state control for inciting violence, as a countermeasure against such attacks.The prime minister said on Saturday that most of those killed in a massacre were British, revising an earlier death toll from 39 to 38 and saying the previous figure had also included the dead gunman.