SRI Lankan officials failed to heed warnings from intelligence agencies about the threat of an attack by a domestic radical Muslim group that officials have blamed for the Easter Sunday bombings which killed more than 200 people, a government minister has said.

The co-ordinated bombings that ripped through churches and luxury hotels were carried out by seven suicide bombers from a militant group called National Thowfeek Jamaath, health minister Rajitha Senaratne said.

International intelligence agencies warned of the attacks several times starting on April 4, Senaratne said.

On April 9, the defence ministry wrote to the police chief with intelligence that included the group's name, according to Senaratne.

On April 11, police wrote to the heads of security of the judiciary and diplomatic security division, he added.

It was not immediately clear what action, if any, was taken in response. Authorities said little was known about the group except that its name had appeared in intelligence reports.

Because of political dysfunction within the government, Seranatne said, Prime Minister Ranil Wickremesinghe and his cabinet were kept in the dark about the intelligence until after the attacks.

President Maithrela Sirisena, who was out of the country at the time of the attacks, ousted Wickremesinghe in late October and dissolved the Cabinet.

The Supreme Court eventually reversed his actions, but the prime minister has not been allowed into meetings of the Security Council since October.

All of the bombers were Sri Lankan citizens, but authorities suspect foreign links, Senaratne said.

Earlier, Ariyananda Welianga, a government forensic crime investigator, said an analysis of the attackers' body parts made clear that they were suicide bombers. He said most of the attacks were carried out by a single bomber, with two at the Shangri-La Hotel in the capital, Colombo.

The National: Sri Lankan police inspect the site of the explosion at the Shangri-La Hotel, ColomboSri Lankan police inspect the site of the explosion at the Shangri-La Hotel, Colombo

The bombings, Sri Lanka's deadliest violence since a devastating civil war ended a decade ago on the island nation, killed at least 290 people and wounded more than 500 others, police spokesman Ruwan Gunasekara said.

Sri Lankan police investigating the bombings are examining reports that intelligence agencies had warnings of possible attacks, officials said.

Two government ministers have alluded to intelligence failures.

Telecommunications Minister Harin Fernando tweeted: "Some intelligence officers were aware of this incidence. Therefore there was a delay in action. Serious action needs to be taken as to why this warning was ignored."

He said his father had heard of the possibility of an attack as well and had warned him not to enter popular churches.

And Mano Ganeshan, the minister for national integration, said his ministry's security officers had been warned by their division about the possibility that two suicide bombers would target politicians.

The police's Criminal Investigation Department, which is handling the investigation into the blasts, will look into those reports, Gunasekara said.

Cardinal Malcolm Ranjith, the Archbishop of Colombo, said the attacks could have been thwarted.

"We placed our hands on our heads when we came to know that these deaths could have been avoided. Why this was not prevented?" he said.

Earlier, defence minister Ruwan Wijewardena described the blasts as a terrorist attack by religious extremists, and police said 13 suspects had been arrested, though there was no immediate claim of responsibility. Wijewardena said most of the bombings were believed to have been suicide attacks.

But officials have yet to say who they believe is behind the attacks.

The Tamil Tigers, once a powerful rebel army known for its use of suicide bombers, was crushed by the government in 2009, and had little history of targeting Christians.

While anti-Muslim bigotry has swept the island in recent years, fed by Buddhist nationalists, the island also has no history of violent Muslim militants.

The explosions – mostly in or around Colombo – collapsed ceilings and blew out windows, killing worshippers and hotel guests in one scene after another of smoke, soot, blood, broken glass, screams and wailing alarms.

A morgue worker in the town of Negombo, outside Colombo, where St Sebastian's Church was targeted, said many bodies were hard to identify because of the extent of the injuries.

The National: Lalitha, centre, mourns the death of her 12-year-old niece, Sneha Savindi, who was a victim of Sunday's bombingsLalitha, centre, mourns the death of her 12-year-old niece, Sneha Savindi, who was a victim of Sunday's bombings

Lakmal, a 41-year-old businessman in Negombo who declined to give his last name, went with his family to St Sebastian's for Easter Mass.

He said they all escaped the blast unscathed, but he remains haunted by images of bodies being taken from the sanctuary and tossed into a truck.

At the Shangri-La Hotel, a witness said "people were being dragged out" after the blast.

"There was blood everywhere," said Bhanuka Harischandra, a 24-year-old from Colombo and founder of a tech marketing company, who was heading to the hotel for a meeting when it was bombed.

"People didn't know what was going on. It was panic mode."

Most of those killed were Sri Lankans. But the three bombed hotels and one of the churches, St Anthony's Shrine, are frequented by foreign tourists, and Sri Lanka's foreign ministry said the bodies of at least 27 foreigners from a variety of countries had been recovered.

The US said "several" Americans were among the dead, while the UK, India, China, Japan, Portugal and Australia said they, too, had lost citizens.

The streets were largely deserted on Monday morning, with most shops closed and a heavy deployment of soldiers and police. Stunned clergy and onlookers gathered at St Anthony's Shrine, looking past the soldiers to the stricken church.

The Sri Lankan government initially lifted a curfew that had been imposed during the night but reinstated it on Monday afternoon.

Most social media remained blocked on Monday after officials said they needed to curtail the spread of false information and ease tension in the country of about 21 million people.

Prime Minister Wickremesinghe said he feared the massacre could trigger instability in Sri Lanka, and vowed to "vest all necessary powers with the defence forces" to take action against those responsible.

Six nearly simultaneous blasts took place in the morning at the shrine and the Cinnamon Grand, Shangri-La and Kingsbury hotels in Colombo, as well as at two churches outside Colombo.

A few hours later, two more blasts occurred just outside Colombo, one at a guesthouse where two people were killed, the other near an overpass.

Three police officers were killed during a search at a suspected safe house on the outskirts of Colombo when its occupants apparently detonated explosives to prevent arrest, authorities said.

They said a large bomb had been found and defused late on Sunday on an access road to the international airport.

Air Force Group Captain Gihan Seneviratne said on Monday that authorities had found a pipe bomb filled with 110lb (50kg) of explosives. It was large enough to have caused damage to a 400-yard (400m) radius, he said.

Harischandra, who witnessed the attack at the Shangri-La Hotel, said there was "a lot of tension" after the bombings, but added: "We've been through these kinds of situations before."

He said Sri Lankans are "an amazing bunch" and noted that his social media feed was flooded with photos of people standing in long lines to give blood.