At Least 15 Britons Killed in Attack at Tunisia Beach Resort

— -- At least 15 British people were among those killed when a gunman opened fire on a beach in Tunisia, Britain's foreign minister said Saturday.

Tobias Ellwood said the beachfront massacre was "the most significant terrorist attack on the British people" since suicide bombers struck the London transport network in 2005, killing 52 and wounding hundreds. The gunman killed 39 people Friday at the Hotel Imperial Marhaba in Sousse, a popular resort town on the northeast coast of Africa.

Ellwood added the number of British people killed Friday "may well rise."

The gunman hid his rifle under an umbrella before he opened fire, security official Rafik Chelli told Tunisia radio station Mosaique FM. He then entered the hotel through the pool and began firing.

The gunman was killed by police after the attack, the country's interior ministry told the Associated Press.

ISIS claimed responsibility for the attack in a statement circulated on social media.

Sousse is one of Tunisia's most popular vacation spots and draws visitors from across Europe -– particularly France. The nationalities of the victims were still being determined, officials said Friday, but a security official told Mosaique FM that many of those killed were foreigners.

The hotel had 565 guests at the time of the attack, mostly from the United Kingdom and central Europe, RIU Hotels & Resorts said in a statement on its website.