ISIS claims Tunisia beach resort massacre

The number of Britons confirmed to have died in a terrorist attack in a Tunisian seaside resort has risen to 15 and could rise further, the British Foreign Office said on Saturday.

At least 38 dead were killed in the attack claimed by the Islamic State of Iraq and Syria (ISIS) on Saturday; a massacre considered the worst attack in the country's recent history.

In a statement released on Twitter, ISIS said the gunman, who they identified as Abu Yahya al-Qayrawani, was a "solider of the caliphate" and most of those killed were "subjects of states that make up the crusader alliance fighting the state of the caliphate".

"Our brother, the soldier of the Caliphate, Abu Yihya al-Kairouni, reached his target the Imperial hotel despite the security measures," the statement said. It said he had attacked a "bordel" and killed 40 "infidels.”

Most of those killed in the massacre were British, the Tunisian Prime Minister Habib Essid said on Saturday.

"The majority are British," he told a news conference. "After come the Germans, then the Belgians and then other nationalities," he said, adding that there were also French among the victims. Essid also revised an earlier death toll from 39 to 38, saying the previous figure had also included the dead gunman who had attacked the popular beach resort of Port el Kantaoui.

Junior foreign minister Tobias Ellwood said Saturday said Friday's massacre, in which 38 people died, was "the most significant terrorist attack on the British people" since the London bombings on July 7, 2005.

Essid said reserve troops would be called up to reinforce security at "sensitive sites... and places that could be targets of terrorist attacks" after the massacre, the second against tourists in Tunisia this year.

The "exceptional plan to better secure tourist and archaeological sites" will include "deploying armed tourist security officers all along the coast and inside hotels from 1 July," he added.

The premier also announced that a national anti-terrorism congress would be held in September and that financial rewards would be given to anyone who comes forward with information leading to the capture of any terrorists.

The government also announced plans within a week to close down 80 mosques that remain outside state control for inciting violence, as a countermeasure after the hotel attack.