SOUSSE, Tunisia >> Tunisia’s prime minister announced on Saturday a string of new security measures including closing renegade mosques and calling up army reservists as thousands of tourists fled the North African country in wake of its worst terrorist attack ever.

Tourists crowded into the airport at Hammamet near the coastal city of Sousse where a young man dressed in shorts on Friday pulled an assault rifle and grenades out of his beach umbrella and killed 38 people, mostly tourists. At least 15 victims were British, according to the U.K. foreign minister.

Tobias Ellwood warned that the toll "may well rise" and described it as the most "significant terrorist attack on the British people," since the July 2005 bombings of the London transport network that killed 52 people.

In the early hours of the morning Saturday, a visibly exhausted Prime Minister Habib Essid called for everyone to work against terrorism and promised financial rewards for information leading to arrests.

"The fight against terrorism is a national responsibility," he said. "We are at war against terrorism which represents a serious danger to national unity during this delicate period that the nation is going through."

The attack came the same day that a suicide bomber killed 27 people in a Shiite mosque in Kuwait and a man in France ran his truck into a warehouse and hung his employer’s severed head on the gate.

Essid announced the call-up of army reservists and said they would be deployed in tourist sites around the country and inside hotels, while he called on the hotels themselves to do more to enforce security.

He also said that political parties and associations espousing radical ideas with suspicious funding would be closed down and around 80 mosques known for extremist preaching would be shut.

The government has been criticized for its lackluster anti-terror measures, especially since 22 people were killed by gunmen at the national museum in March. There was also been a failed suicide bomb attack in Sousse in 2013.

The attacker, who was killed by security forces, was identified as Seifeddine Rezgui, a young student at Kairouan University.

Witnesses said the young man used grenades in the attack as he moved methodically from the beach to the pool and then into the lobby.

"When he came he tossed a grenade and we saw only black — it was smoky," Imen Belfekih, who works for the Imperial Marhaba Hotel where the attack took place, told The Associated Press. Other accounts from survivors also describe hearing explosions in addition to the gunfire during the attack.

A tweet from the Islamic State group claimed responsibility for the attack and gave his jihadi pseudonym of Abu Yahya al-Qayrawani, according to the SITE intelligence group.

The attack in Kuwait was also claimed by the Islamic State and on Saturday thousands of people took part in a mass funeral procession. Police said they are interrogating a number of suspects with possible links to the bombing, which was claimed by an affiliate of the Islamic State group.

In southeastern France, one of the four suspects detained over the explosion and beheading has been released, while the suspected killer was not speaking to investigators, a spokeswoman for the Paris prosecutor’s office said Saturday.

At the Imperial Marhaba vans and buses were carrying away tourists on Saturday. While the hotel wasn’t actually closing down, the tour operators had urged everyone to leave, the director said.

"We may have zero clients today but we will keep our staff," said Mohammed Becheur, adding the 370-room hotel had been at 75 percent occupancy before the attack.

On the beach, there was as scattering of tourists while police on horses rode along the beach and their security boats patrolled along the coast.

Tourism is a key part of Tunisia’s economy and had already fallen some 25 percent after the March museum attack.

"It’s really sad but what can you do, for everyone, for the tourists, for the people who died, for their families," said Belgian tourist Clause Besser, as he recovered in the hospital from a gunshot wound he received fleeing from the attacker. "For me, somehow, with a bullet in the leg, it’s not a catastrophe. For those who died or were injured for life, it’s something else."

British travel companies Thomson and First Choice said they are flying back thousands of tourists from Tunisia on Saturday and are cancelling all flights to the country in the coming week. Tourist flights from Ireland to Tunisia have continued in the wake of the attack, but travel agents are offering full refunds for those canceling.

Slovakia has sent a plane to evacuate about 150 of its citizens who are in Tunisia, according to the Foreign Ministry. Scandinavia tour operators say they are flying thousands of people out of Tunisia over the next days.

"Since yesterday, there have been about 15 extra flights scheduled at the airport with nine already leaving carrying 1,400 passengers," said Mohamed Walid Ben Ghachem, director of the Enfidha-Hammamet Airport.

Waiting for his flight, British tourist Matthew Price said that he was cutting his weeklong vacation short by three days. Even though it was his third visit to Tunisia, he expected it to be his last.

"It’s the first time I’ve ever been on holiday and feared for my life. So obviously you can’t come back somewhere it’s not safe," he said.

The Tunisian Health Ministry has said that a Belgian and a German were killed in addition to Britons. Ireland’s government said an Irish nurse was also among those who were killed.

Relatives and family friends say Lorna Carty was fatally shot as she sunbathed. She and her husband, Declan, had received the holiday as a present to help Declan Carty relax following his recent heart surgery. Family friends speaking to the couple’s two children said Lorna Carty went ahead of her husband to the beach, where she was shot and she was later pronounced dead at a hospital.

Of the wounded, 24 were Britons, seven Tunisian, three Belgian as well as a German, Russian and Ukrainian and one unidentified, said the Health Ministry. Two were still in critical condition.

While many tourists have left, Welsh couple Angela Chambers and Peter Phillips said from the beach near where the attack took place that they had declined their tour operator’s offer to take them to the airport last night.

"We want to see our holiday out," Phillips said.