The Islamic State jihadist group claimed responsibility Saturday for the tourist hotel attack in eastern Tunisia that killed 39 people, including 15 British tourists.

The SITE Intelligence Group said that the Islamic extremist group identified the gunman as Abu Yahya al-Qayrawani. ISIS-supported Twitter accounts have also released photographs of gunman, according to the Daily Mail.

"Our brother, the soldier of the Caliphate, Abu Yihya al-Kairouni, reached his target the Imperial hotel despite the security measures," the jihadist group said, according to i24news. The statement further said that the soldier had attacked a "bordel" and killed 40 "infidels."

Tunisian interior ministry sources, however, identified the gunman as Saifeddine Rezgui, a 23-year-old electrical engineering student, and said he was not in their terrorist watch list, Reuters reported.

At least 39 people, mostly foreign tourists, were shot dead at the Tunisian resort of Port el Kantaouni in the eastern coastal city of Sousse after a gunman pulled out a gun hidden inside a beach umbrella.  

"He entered by the beach, dressed like someone who was going to swim, and he had a beach umbrella with his gun in it. Then when he came to the beach he used his weapon," Tunisian Secretary of State for Security Rafik Chelly told Mosaique FM, according to Arab Time.

The British Foreign Office confirmed the death of 15 British nationals in Friday's attack and warned that the death toll was likely to increase, BBC News reported. 

"I'm afraid that the British public need to be prepared for the fact that many of those killed were British," British Prime Minister David Cameron said on Saturday.

Tunisia's Prime Minister Habib Essid has announced a string of tough measures to counter extremists, including closing some 80 uncontrolled mosques in the wake of its worst terrorist attack, Times of Malta reported.