Sri Lanka's government has acknowledged "major" lapses over its failure to prevent the attacks on Easter Sunday that killed more than 350 people, despite prior intelligence warnings.

Recriminations have flown since Islamist suicide bombers blew themselves up in packed churches and luxury hotels on Sunday, in attacks claimed by the so-called Islamic State group.

Overnight, security forces using newly granted powers under the country's state of emergency arrested 18 more suspects in connection with the attack, as the toll rose to 359.

The United Nations said at least 45 children are among the dead as well as a number of foreign national.

Police have so far arrested 58 people, all Sri Lankans, and security remains heavy, with bomb squads carrying out several controlled explosions of suspect packages today.

Read More:

Sri Lanka PM believes attacks had Islamic State links
Sri Lanka imposes emergency after bombings
President expresses solidarity with Sri Lanka over attack 

But the government faces anger over revelations that specific warnings about an attack went ignored.

Sri Lanka's police chief issued a warning on 11 April that suicide bombings against "prominent churches" by local Islamist group, National Thowheeth Jama'ath were possible and alerts had been given by a foreign intelligence agency.

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.

But that information was not shared with the prime minister or other top ministers, the government says.

"It was a major lapse in the sharing of information," deputy defence minister Ruwan Wijewardene conceded at a press conference.

"The government has to take responsibility."

CCTV footage has emerged showing one of the attackers calmly patting a child on the head and shoulder moments before he walked into the packed St Sebastian's church and detonated his bomb among those attending mass.

President Maithripala Sirisena, who is also defence and law and order minister, pledged yesterday to make "major changes in the leadership of the security forces in the next 24 hours".

"The restructuring of the security forces and the police will be completed within a week," he said.

New details emerged about some of the bombers, with Mr Wijewardene saying one had studied in Britain and then did post-graduate studies in Australia before returning to Sri Lanka.

"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 added.

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

Experts say the bombings bear many of the hallmarks of IS attacks, and the government has suggested local militants could not have acted alone.

But it has not yet officially confirmed any IS role in the blasts against three churches packed with Easter worshippers and three high-end hotels.

A search is under way for other suspects linked to the blasts, including the head of a local Islamist group believed to have played a key role in the attacks.

The government has said the National Thowheeth Jama'at group was behind the attack, perhaps with international help, and its leader Zahran Hashim remains unaccounted for.

He appears to be among eight people seen in a video released by IS yesterday, leading seven others in a pledge of allegiance to IS chief Abu Bakr al-Baghdadi.

It was not yet clear whether Hashim was among the suicide attackers or had escaped after the blasts.

Government officials have said they cannot rule out further attacks while suspects remain at large.

In all, nine people are believed to have blown themselves up in Sunday, either during attacks or when police attempted to arrest them.

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.

Sources close to the investigation said two more people - a man and a woman - blew themselves up at another location as security forces launched a raid. Those blasts killed three police.

Work was continuing to identify foreign victims in the blasts.

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

Among the British victims were Anita Nicholson, her son Alex, 14, and 11-year-old daughter Annabel, who died when a suicide bomber struck as they ate breakfast at the Shangri-La hotel in Colombo.

London teenagers Daniel Linsey, 19, and his younger sister Amelie, 15, were also killed at the Shangri-La hotel on the final day of their holiday, according to media reports quoting relatives.

Their father Matthew Linsey survived and described to The Times desperately trying to revive his son in the aftermath of the attack.

"You can't describe how bad it was," he told the newspaper. "People were screaming. I was with my children. I couldn't tell whether they were all right, it was dark."

Danish billionaire Anders Holch Povlsen lost three of his four children in the attacks.

Mr Povlsen, his wife Anne and their four children were in Sri Lanka on holiday.

Considered to be Denmark's richest man, the 46-year-old is the main shareholder in the online fashion retailer ASOS, as well as the owner of clothing group Bestseller.

Eleven-year-old Kieran Shafritz de Zoysa was killed at the breakfast buffet of a hotel while on holiday with his mother, who suffered minor injuries in the blast.

Kieran was to spend the summer travelling before returning his studies at the prestigious Sidwell Friends School in Washington, his father Alex Arrow told ABC News.

"We should know what the world lost, what they took from the world," Mr Arrow said. "A brilliant mind who ... won't make it to his 12th birthday."

Meanwhile, the US Ambassador to Sri Lanka said they had no prior knowledge of the suicide bomb attacks, but now believes there is ongoing terrorism plotting in the country.

"We had no prior knowledge of these attacks," Ambassador Alaina Teplitz told reporters in Colombo.

"We believe there are ongoing terrorist plots. Terrorists can strike without warning. Typical venues are large gatherings, public spaces."