Police officers surround a man (C) suspected to be involved in an attack on a beachside hotel in Sousse, Tunisia June 26, 2015.[Photo/Agencies]

CAIRO - Militant group Islamic State claimed responsibility for the Tunisia tourist hotel attack on Friday that killed 39 people including Britons and Germans, according to a statement on a Twitter account.

"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'.

Related: Gunman kills 39 at Tunisian beachside hotel

A gunman disguised as a tourist opened fire at a Tunisian hotel on Friday with a rifle he had hidden in an umbrella, killing 39 people including Britons, Germans and Belgians as they lounged at the beach in an attack claimed by Islamic State.

Terrified tourists ran for cover after the gunfire and an explosion erupted at the Imperial Marhaba in Sousse, 140 km (90 miles) south of the capital Tunis, before police shot the gunman dead, witnesses and security officials said.

The bodies of several tourists lay crumbled where they fell in the sand, covered with yellow towels and blankets among the plastic white sunbeds. Blood smeared stone steps leading from the hotel main area.

"This was always a safe place but today was horror," said an Irish tourist who gave only his first name, Anthony. "He started on the beach and went to the lobby, killing in cold blood."

The attack took place during the holy Muslim month of Ramadan, on a day in which a decapitated body daubed with Arabic writing was found in France, a suicide bomber killed two dozen people at a mosque in Kuwait and at least 145 civilians were reported killed by Islamic State militants in northern Syria.

In a statement on social media, Islamic State claimed responsibility for the Tunisian attack. It had urged its followers to step up assaults during Ramadan.

Twitter accounts that support Islamic State released three photos they said showed the gunman. A man is seen from behind walking down a street and holding an assault rifle, though his identity and the location are unclear.

"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'.

After pulling out a weapon hidden inside an umbrella, the assailant strolled through the hotel grounds, opening fire left and right at the pool and beach, reloading his weapon several times and tossing an explosive, witnesses said.

A security source said another bomb was found on his body, which lay with a Kalashnikov assault rifle where he was shot.

Rafik Chelli, a senior interior ministry official, said the gunman killed was unknown to authorities and not on any watchlist of potential jihadists. A security source named him as Saifeddine Rezgui, a 23-year-old electrical engineering student.

Local radio said police captured a second gunman, but officials did not immediately confirm the arrest or his role in the attack.

"It was just one attacker," said a hotel worker at the site. "He was a young guy dressed in shorts like he was a tourist himself."