The British government cautioned Sunday that other attacks "are possible" in Tunisia after the mass shooting two days ago that left 38 people dead including at least 15 Britons.

Interior minister Theresa May said there had been no change in the British toll, the worst in a terror attack since the 2005 London bombings, but told the BBC: "We are expecting that to rise."

Three members of the same family were among at least 15 Britons killed in the Tunisia attack, reports said Sunday, in Britain's worst loss of life in a terror incident since the 2005 London bombings.

As ministers warned the toll would likely rise, details began to emerge of those gunned down in Friday's Islamist massacre at a popular beach resort.

Iran has condemned the jihadist attacks in Tunisia, Kuwait and France, saying they were "contrary to the teachings of Islam," in statements released Saturday by the foreign ministry.

The killings were not apparently coordinated, but the Islamic State group claimed the atrocities in Tunisia and Kuwait, just days before the first anniversary of it declaring a "caliphate" in Iraq and Syria.

Thousands of scared foreign holidaymakers were being flown from Tunisia on Saturday after an Islamist gunman killed 38 people, most of them British tourists, at a beach resort.

The Islamic State jihadist group, which controls swathes of Iraq and Syria, claimed responsibility for the attack, the deadliest in Tunisia's recent history.

Security agencies have intensified their measures throughout Lebanon in light of Friday's terror attacks in Kuwait and Tunisia, reported As Safir newspaper on Saturday.

A security source told the daily that the agencies went on alert in anticipation of the occurrence of new terrorist attacks.

British holidaymakers told how they fled in panic from Friday's gun massacre on a Tunisian beach resort, screaming and running for cover when they realized they were under attack.

Five Britons were among the 37 dead in the shootings in Port el Kantaoui outside the town of Sousse.

Italy raised its terror alert level on Friday after deadly attacks in France, Tunisia and Kuwait.

"No country is without risk, we have raised the level of alert to re-sensitize those units charged with protecting sensitive places," Interior Minister Angelino Alfano said.

Spain raised its terror alert level from medium to high on Friday after deadly attacks in France, Tunisia and Kuwait, its Interior Minister Jorge Fernandez Diaz said.

"Considering the proximity of our country to the places where some of these attacks took place, it has been proposed to raise the anti-terrorist alert" from three out of five, or 'medium', to four or 'high', he told a news conference. 

Muslim clerics condemned three militant attacks in Tunisia, Kuwait and France on Friday that killed dozens of people including holidaymakers.

Al-Azhar, a leading Sunni Muslim institution based in Egypt, said the "heinous" shooting at a Tunisian coastal resort which killed 28 people, mostly Europeans, was a "violation of all religious and humanitarian norms."

The United States condemned "heinous attacks" in France, Kuwait and Tunisia on Friday but said it had seen no evidence that they were directly coordinated.

"Our thoughts and prayers are with the victims of these heinous attacks, their loved ones, and the people of all three countries," the White House said.