The number of people dead after the Sri Lanka bombings has risen to 359 and more than 60 suspects have been arrested in connection with the terror attacks.

It comes as Islamic State has formally claimed responsibility for the Easter bomb attacks in what officials believe was retaliation for the Christchurch mosques massacre.

IS claimed responsibility Wednesday morning via its Amaq news agency, naming who it said were the seven attackers that carried out the attacks.

IS later released a video showing eight men, seven of whom were masked, pledging allegiance to IS leader Abu Bakr al-Baghdadi.

The footage purported to show an unmasked Mohammed Zahran, also known as Zahran Hashmi, who Sri Lanka says led the Easter attack.

It gave no further evidence to support its claim of responsibility.

The IS video purported to show bombing ringleader Mohammed Zahran. Photo: SITE

Sri Lankan authorities earlier said two domestic Islamist groups with suspected links to foreign militants were believed to have been behind the seven suicide bombings at three churches and four hotels.

About 500 people were also wounded in the attacks.

Christchurch revenge attack

New Zealand Prime Minister Jacinda Ardern said Wednesday that her government is not aware of any intelligence suggesting the Sri Lanka bombings were in retaliation for last month’s deadly shootings at mosques in Christchurch.

Sri Lankan junior minister for defence, Ruwan Wijewardene, told Parliament  investigators on Tuesday that he believed the motive for the attacks was revenge for the March 15 killing of 50 people at two mosques in the New Zealand city.

“We haven’t received anything officially nor have we received any intelligence reports that corroborate what has been said in Sri Lanka,” Ms Ardern told reporters in Auckland.

“Sri Lanka will be in the very early stages of its investigations. So we are simply stepping back and allowing them to undertake those, but we have nothing at this stage to corroborate what is being said,” she added.

Mr Wijewardene did not elaborate on why Sri Lankan authorities believed there was a link to the New Zealand killings, perpetrated by a lone gunman.

He said two Sri Lankan Islamist groups – the National Thawheed Jama’ut and Jammiyathul Millathu Ibrahim – were responsible for the blasts, which detonated during Easter services and as hotels served breakfast.

Specific warning

Sri Lankan intelligence officials had reportedly received an explicit warning hours earlier from India that attacks by Islamists were imminent. It was not clear what action, if any, was taken.

President Maithripala Sirisena said he would change the heads of the defence forces following their failure to act on the intelligence.

“I will completely restructure the police and security forces in the coming weeks. I expect to change the heads of defence establishments within the next 24 hours,” Mr Sirisena said in an address to the nation on Tuesday night.

“The security officials who got the intelligence report from a foreign nation did not share it with me. I have decided to take stern action against these officials.”

Footage on CNN showed what it said was one of the bombers wearing a heavy backpack. The man patted a child on the head before entering the Gothic-style St Sebastian church in the coastal city of Negombo, north of Colombo.

Footage of one of the alleged bombers was captured on CCTV. Photo: CNN

Prime Minister Ranil Wickremesinghe told a news conference investigators were making progress in identifying the perpetrators.

“We will be following up on IS claims, we believe there may be some links,” he said.

Indian intelligence officers contacted their Sri Lankan counterparts two hours before the first attack to warn of a specific threat on churches, one Sri Lankan defence source and an Indian government source were quoted by Reuters as saying.

The first six bombs –  on three churches and three luxury hotels – exploded within 20 minutes of each other. Two more explosions – at a downmarket hotel and a house in a suburb of the capital, Colombo – took place in the early afternoon.

Mr Wickremesinghe said an attempted attack on another hotel had failed.

Sri Lankan government and military sources said a Syrian had been detained among 40 people being questioned over the bombs.

Security forces were on alert for more attacks and the government imposed emergency rule giving police extensive powers to detain and interrogate suspects. An overnight curfew has also been in place since Sunday.

-with AAP