Sri Lankan security forces detained 18 new suspects in the Easter bombings that killed more than 350 people, as the government admitted Wednesday that "major" intelligence lapses had led to a failure to prevent the attacks.

The Islamic State group's claim of responsibility for Sunday's strikes on three hotels and three churches increased pressure over ignored warnings of the bombings, and the government was expected to announce sackings in the security forces and police leadership.

The 18 new arrests were made in night-time raids using emergency powers, said police spokesman Ruwan Gunasekera.

Fifty-eight people are now in custody and more arrests are expected in coming days, deputy defence minister Ruwan Wijewardene told a press conference.

The government has said more attacks are possible and in a sign of the heightened tensions, authorities carried out at least two controlled explosions of a suspect package and vehicle on Wednesday.

Wijewardene also gave new details on the bombers. One had studied in Britain and then did post-graduate studies in Australia before returning to Sri Lanka. Others were also believed to have travelled abroad, he told a press conference.

"Most of them are well-educated and come from middle, upper-middle class families, so they are financially quite independent and their families are quite stable financially, that is a worrying factor in this," the minister said.

- IS claim -

Amid the frantic hunt for suspects, Islamic State claimed responsibility for the strikes and released photos and video of the men it said were involved in the attacks.

Seven of the eight men in the images had their faces covered. Islamic State group called the blasts "blessed attacks" on the "infidel holiday".

The authenticity of the images could not be independently verified but experts said the attacks bear many hallmarks of a coordinated Islamic State operation.

The government has blamed a local Islamist group, National Thowheeth Jama'ath (NTJ), and said they were investigating whether the bombers had foreign help.

The deputy defence minister said it was believed the suicide bombers came from an NTJ "splinter group" whose leader had killed himself.

In the Islamic State video, a man believed to be the leader of the NTJ, Zahran Hashim, directs a group in a pledge of allegiance to IS chief Abu Bakr al-Baghdadi.

Sri Lankan police sources have told AFP that two Muslim brothers, sons of a wealthy Colombo spice trader, blew themselves up at the Shangri-La and the Cinnamon Grand hotels.

Their father is now one of the 58 in custody.

The Kingsbury hotel in the capital was the last one hit. A fourth planned attack on a hotel failed, authorities said. The would-be attacker was followed back to a Colombo lodge, where he blew himself up, killing two people.

The funerals of dozens more victims were held Wednesday but the government became embroiled in recriminations over warnings about the attacks up to two weeks before they were carried out.

- Leadership cull -

President Maithripala Sirisena, who is also defence and law and order minister, has pledged "major changes" to the security forces and police leadership.

Sri Lanka's police chief issued a warning on April 11 that NTJ suicide bombings were possible and alerts had been given by a foreign intelligence agency.

Prime Minister Ranil Wickremesinghe has acknowledged the information was not passed to his office or other top ministers.

"It was a major lapse in the sharing of information," the deputy defence minister said. "The government has to take responsibility."

CNN reported that Indian intelligence services had passed on "unusually specific" information in the weeks before the attacks, some of it from an IS suspect in their custody.

The US ambassador to Sri Lanka has denied that her government had prior knowledge of the planned attacks.

A US FBI team is now in Sri Lanka, Wijewardene said, and Britain, Australia and the United Arab Emirates have all offered intelligence help.

Sri Lankan Muslim leaders said they had many several complaints to police about Hashim's activities. His whereabouts after the bombings were unknown however.

- Identifying the dead -

Work was continuing to identify foreign victims in the blasts.

A Danish billionaire lost three of his children in the attacks, a spokesman for his company said.

Eight Britons, 10 Indians, four Americans and nationals from Turkey, Australia, Japan and Portugal were also reported killed.

The United Nations said at least 45 children, Sri Lankans and foreigners, were among the dead.