Tunisia has launched a security clampdown after Sousse attack in which 38 people were killed, PM Habib Essid has announced.

Habib Essid said army reservists would be deployed to archaeological sites and resorts.

About 80 mosques accused of “spreading venom” will close within a week, he said.

Authorities say most of the victims in yesterday’s attack near Sousse were Britons. The gunman was shot dead.

Officials say he was a student not previously known to authorities.

Five Britons were confirmed dead and the Foreign Office warned the death toll could rise.

Tunisians, Germans, Belgians, French and at least one Irish citizen were also among those killed in the attack, which was claimed by Islamic State (ISIS). Thirty-six people are injured, officials say.

It was the second major attack on tourists in Tunisia since March, when militants killed 22 people, mainly foreigners, at Bardo museum in Tunis.

Speaking at a news conference in Tunis, Habib Essid said some mosques outside government control were spreading “their propaganda and their venom to promote terrorism” and would be shut down.

Habib Essid also vowed to act against parties and groups “acting outside the constitution” – such action could range from a warning to closure.

He said army reservists would be deployed at sensitive sites to ensure such attacks could not happen again.

“The country is under threat; the government is under threat,” Habib Essid said.

“Without the co-operation of everyone and a show of unity, we cannot win this war. We have won some battles and lost others, but our objective is to win the war.”

Security officials said the attacker, who had posed as a swimmer but was carrying a rifle under a parasol, started shooting on the beach before entering the Hotel Imperial Marhaba, continuing to shoot.

Local media reported a second suspected attacker had been arrested, but this has not been confirmed.

ISIS had urged followers to step up assaults during the holy Muslim month of Ramadan.

Many of the tourists in hotels affected by the attack left Tunisia overnight, with tour operators arranging special flights to take them home.

Friday’s attack was the deadliest in Tunisia’s recent history. The country has seen militant Islamists gain strength since the overthrow of long-serving ruler Zine al-Abidine Ben Ali in in 2011.

Democratic elections after Zine al-Abidine Ben Ali’s removal saw the moderate Islamist Ennahda party take power before the secularist Nidaa Tounes government won a parliamentary poll in October.

However, neither party has been able effectively to combat Islamist violence – which has been made worse by conflict in neighboring Libya and by Tunisian fighters returning home after joining Islamist campaigns in Iraq and Syria.