Isis has claimed responsibility for bombings that left more than 300 people dead in Sri Lanka, releasing a video of the alleged attackers pledging allegiance to its leader.

In a statement released by its Amaq propaganda agency, the group said it was targeting citizens of countries bombings its territories and Christians.

“A security source told Amaq agency the perpetrators of the attack targeting the citizens of [US-led] coalition countries and Christians in Sri Lanka were Islamic State fighters,” it said.

The wording of Isis's claim is similar to that for previous atrocities that were thought to be inspired by the group but not directly orchestrated by it.

It came after Isis supporters circulated unverified photographs claiming to show three of the suicide bombers who targeted churches and hotels on Easter Sunday.

Sri Lanka: Scene of the bomb attacks and aftermath Show all 38 1 /38 Sri Lanka: Scene of the bomb attacks and aftermath Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan Special Task Force officers raid a house following an explosion in capital Colombo AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath A blood-spattered statue of Jesus Christ while crime scene officials inspect the site of a bomb blast, as the sun shines through the blown-out roof, inside St Sebastian's Church in Negombo Reuters Sri Lanka: Scene of the bomb attacks and aftermath A police officer inspects the damage after a bombing at the Shangri-La Hotel in Sri Lankan capital Colombo AP Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan soldiers look on inside the St Sebastian's Church at Katuwapitiya in Negombo following a bomb blast during the Easter service AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath A woman cries next to a coffin of a bomb blast victim. A series of eight devastating bomb blasts ripped through high-end hotels and churches holding Easter services in Sri Lanka on April 21, killing more than 300 people AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath People who live near the church that was attacked the day before, leave their houses as the military try to defuse a suspected van before it exploded in Colombo Reuters Sri Lanka: Scene of the bomb attacks and aftermath Shoes and belongings of victims are collected as evidence at St Sebastian's Church AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan police were forded to clear an area while Special Task Force Bomb Squad officers inspected the site of an exploded van near a church the day after it was attacked Reuters Sri Lanka: Scene of the bomb attacks and aftermath The Kingsbury Hotel in Colombo, Sri Lanka following a bombing AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Queues at the National Blood Bank in Colombo, Sri Lanka after a request for blood to aid in recovery efforts Mr Lavasquabble/Twitter Sri Lanka: Scene of the bomb attacks and aftermath Medical response team work to remove dead bodies from the scene of bombings in Sri Lankan capital Colombo AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan security personnel inspect the debris of a car after it exploded when police tried to defuse a bomb AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Security personnel work at the scene of a bombing at a church in Batticaloa in Sri Lanka AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath A woman is helped near St Anthony's Shrine after a blast in Colombo, Sri Lanka, on Sunday 21 April 2019. At least 130 people were killed and more than 500 hospitalised after near-simultaneous explosions hit three hotels and three churches on Easter Sunday. AP Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan police officers clear the road as an ambulance drives through carrying injured of church blasts in Colombo, Sri Lanka, on Sunday 21 April 2019 AP Sri Lanka: Scene of the bomb attacks and aftermath A Sri Lankan soldier stands guard near a car explosion AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath A woman prays at St. Sebastian's Church in Negombo AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Priests stand inside St Anthony's Shrine, Kochchikade church on the outskirts of Colombo after a bombing Reuters Sri Lanka: Scene of the bomb attacks and aftermath A Sri Lankan woman living near St. Anthony's shrine runs for safety with her infant after police found explosive devices in a parked vehicle in Colombo AP Sri Lanka: Scene of the bomb attacks and aftermath People react during a mass for victims, two days after a string of suicide bomb attacks on churches and luxury hotels Reuters Sri Lanka: Scene of the bomb attacks and aftermath Security personal react as a device was detonated in a controled explosion in a van near the St Anthony's Church EPA Sri Lanka: Scene of the bomb attacks and aftermath Security forces inspect the scene of a bombing at the Shangri-La Hotel in Sri Lankan capital Colombo Getty Sri Lanka: Scene of the bomb attacks and aftermath The Kingsbury hotel in Colombo, Sri Lanka after a bombing attack Getty Sri Lanka: Scene of the bomb attacks and aftermath The damaged interior of a church in Negombo, Sri Lanka following a bombing attack Reuters Sri Lanka: Scene of the bomb attacks and aftermath Police and security personnel stand guard outside the Shangri-La Hotel in Sri Lankan capital Colombo following a bombing attack EPA Sri Lanka: Scene of the bomb attacks and aftermath Debris is seen at St Anthony's Church in this still image from video footage after explosions hit churches and hotels in Colombo, Sri Lanka, on 21 April 2019 Derana TV/via Reuters Sri Lanka: Scene of the bomb attacks and aftermath Damage inside a church following a bombing attack AP Sri Lanka: Scene of the bomb attacks and aftermath A Sri Lankan Special Task Force officer scales a house during a raid following an explosion in capital Colombo AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath A general interior view showing damage after an explosion hit St Anthony's Church in Kochchikade, Colombo, Sri Lanka, on Sunday 21 April 2019 EPA Sri Lanka: Scene of the bomb attacks and aftermath Priests walk into St Anthony's Shrine Kochchikade church after an explosion in Colombo, Sri Lanka, on 21 April 2019 Reuters Sri Lanka: Scene of the bomb attacks and aftermath A Sri Lankan police officer stands outside St Anthony's Shrine in Colombo, Sri Lanka, after a blast on Sunday 21 April 2019 AP Sri Lanka: Scene of the bomb attacks and aftermath A shoe of a victim is seen in front of the St Anthony's Shrine, Kochchikade Church Reuters Sri Lanka: Scene of the bomb attacks and aftermath Local and security officials gather at the scene at St Anthony's Church in Kochchikade, Colombo, Sri Lanka EPA Sri Lanka: Scene of the bomb attacks and aftermath An ambulance is seen outside the church premises with gathered security personnel following a blast at St Anthony's Shrine in Kochchikade, Colombo, on Sunday 21 April 2019 AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Armed policemen patrol the area after an explosion hit at St Anthony's Church in Kochchikade in Colombo, Sri Lanka, 21 April 2019 EPA Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan Army soldiers secure the area around St. Anthony's Shrine after a blast in Colombo, Sri Lanka, Sunday 21 April 2019 AP Sri Lanka: Scene of the bomb attacks and aftermath Sri Lankan security personnel keep watch outside the church premises following a blast at the St Anthony's Shrine in Kochchikade, Colombo, on Sunday 21 April 2019 AFP/Getty Sri Lanka: Scene of the bomb attacks and aftermath Police officers inspect the scene after an explosion hit at St Anthony's Church in Kochchikade in Colombo, Sri Lanka, 21 April 2019 EPA

They showed the men posing with a knife and guns in front of Isis’s black flag, captioned with the names Abul Muktar, Abu Ubaida and Abul Barra.

A second and longer Isis statement released later on Tuesday called the atrocities a "blessed invasion" and said both suicide bombs and IEDs were used.

It named seven attackers using kunyas - including three matching the names used on the photos - which are Arabic war names frequently taken by Isis supporters who have fought for the group in Syria and Iraq.

Sri Lanka's prime minister told a press conference that some of the bombers had travelled abroad and returned home, but did not give further details.

Ranil Wickremesinghe warned there could be more bombs and militants "out there", adding: "We will be following up on Isis's claims, we believe there may be links.”

Isis's statement claimed to detail which bomber attacked which target, and said the seventh man "clashed with police and killed three of them" as security forces approached a safe house in Dematagoda.

The terrorist group published a photograph showing seven men with their faces covered, standing in front of an Isis flag with an unmasked eighth man.

He matched the photo of "Abu Ubaida", who Isis said blew himself up at a hotel, and looked similar to local militant leader Zahran Hashmi.

A photo released by Isis's Amaq propaganda agency claiming to show seven suicide bombers who carried out the Sri Lanka attacks

It was not immediately clear why Isis named seven attackers but published a photo of eight men, although authorities said a fourth hotel bombing had failed.

A one-minute video released by Isis showed the same men pledging allegiance to the "caliphate" and its leader Abu Bakr al-Baghdadi, before shouting "Allahu akbar".

The Sri Lankan government had attributed the atrocities to a little-known local Islamist group called National Thowheed Jama’ath, which was previously known for vandalising Buddhist statues.

But the president asked for foreign assistance to track down the bombers’ international links on Monday, because of intelligence reports “indicating that foreign terrorist organisations are behind the local terrorists”.

The targeting of churches and hotels popular with foreign tourists is a technique recently used by Isis and al-Qaeda affiliates.

But attacks organised or coordinated by Isis have been claimed faster, with detail, and sometimes seen the group put out gory videos and footage from the scene.

Isis released its claim shortly after Sri Lanka’s defence minister said the attacks were “carried out in retaliation” for shootings that left 50 Muslims dead at mosques in New Zealand last month.

Security analysts questioned whether an attack of such complexity, seeing multiple perpetrators strike targets in different parts of the country almost simultaneously - with at least nine bombs - could be prepared in little over a month.

Ruwan Wijewardene did not explain the source of the information but blamed “weakness” within Sri Lanka's security apparatus for failing to prevent the bombings.

“By now it has been established that the intelligence units were aware of this attack and a group of responsible people were informed about the impending attack,” he added after a letter warning security services was leaked online. “However, this information has been circulated among only a few officials.”

Sri Lanka prime minister reacts to deadly Easter blasts

Isis issued a call for its followers to “take vengeance” for the Christchurch attacks in an audio recording from its official spokesman last month.

“The scenes of death in the two mosques are enough to wake the sleep and incite the supporters of the caliphate who live there, to take vengeance for their religion and for sons of their Ummah, who are killed everywhere in the world,” it said.

Experts had warned that the Christchurch massacre could be used for “reciprocal radicalisation”, which sees Islamists justify their attacks using Western air strikes and far-right atrocities, and the far-right justify their attacks using Islamist terrorism.

At the time, security services said they feared a “domino effect” in atrocities as Isis sought publicity and momentum following the destruction of its caliphate.

Sri Lankan militants were among the foreigners who travelled to fight for the group in Iraq and Syria.

National Thowheed Jama’ath’s leader, known as Mohammed Zahran or Zahran Hashmi, became known three years ago for incendiary speeches posted online that called for non-Muslims to be eliminated.

Weeks before the attacks, National Thowheed Jama’ath was named in a police advisory to security services warning of a potential threat to Catholic churches.

Telecommunications minister Harin Fernando posted images of the letter, entitled “information on an alleged plan attack” and detailing names and other details, on Twitter.

Ben Nicholson with wife Anita, son Alex, 14, and daughter Annabel, 11. The lawyer's wife and two children were among victims killed in the series of terror attacks in Sri Lanka. (PA)

He called for “serious action” on why the atrocities were not prevented and the prime minister launched an inquiry.

Zahran was personally named in the warning and some international security experts said they had been asked about the group's activity.

Anne Speckhard, director of the International Center for the Study of Violent Extremism, said a Sri Lankan intelligence official approached her at a February conference with concerns about a violent, homegrown jihadi group.

Ms Speckhard said the official feared militants that “would just disappear” when the government tried to crack down on them.

“The intel person kind of came up to me and said, 'You know, we're kind of worried about this new group and there's some activity going. What do you think?'" Speckhard told the Associated Press. “It just kind of blows my mind that's who it was.”

While Sri Lanka is no stranger to sectarian violence, following a 25-year civil war with Tamil separatists that raged until 2009, Islamists have previously been considered a lesser threat than other groups.

Amarnath Amarasingam, a senior research fellow with the Institute for Strategic Dialogue, pointed out that recent new year celebrations and Buddhist religious events passed off safely.

“If this were local Muslim extremist groups who wanted to retaliate against anti-Muslim attacks by Buddhists, I think the targeting would have been a bit different,” he wrote on Twitter.

A total of nine bombings on Sunday killed at least 321 people and injured more than 500.

Sri Lankan officials said two people were involved in the attack at the Shangri-La hotel.

One bomber each attacked the Cinnamon Grand and Kingsbury hotels, and St Anthony's Shrine in Colombo, St Sebastian's church in the city of Negombo and Zion Church in the city of Batticaloa.

Two bombings hours later at a guesthouse and near an overpass on the outskirts of Colombo are still under investigation.