The terrorist group ISIS has claimed responsibility for the bomb attacks on churches and hotels in Sri Lanka that killed at least 321 people on Easter Sunday, but provided no evidence.

In a bulletin posted by its Aamaq news agency, ISIS said: “The perpetrators of the attack that targeted nationals of the countries of the coalitions and Christians in Sri Lanka before yesterday are fighters from the Islamic State,” according to a translation by the Associated Press.

The news came on Tuesday as the Sri Lankan minister of defense Ruwan Wijewardene said the attacks were in retaliation for last month’s terrorist attack against Muslims in Christchurch, New Zealand, in which 50 people were killed. He did not immediately offer any evidence. “The preliminary investigations have revealed that what happened in Sri Lanka was in retaliation for the attack against Muslims in Christchurch,” he told the Sri Lankan parliament.

Later on Tuesday, Sri Lankan Prime Minister Ranil Wickremesinghe warned more explosives were “out there,” acknowledging his security services had failed to act on an advance warning that a terror attack was being planned. He said some security officials would lose their jobs as a result. Wickremesinghe said that an attack at a fourth church had failed.

Sri Lanka first blamed a local Islamic extremist group called National Thowheeth Jama’ath for the attack, but officials said it had received help from an international terrorist organization, according to the New York Times. Sri Lanka has implemented an “emergency law” giving police powers to detain suspects without a warrant, and 40 people have now been arrested in connection to the attacks.

ISIS has now lost all the territory it once held in Iraq and Syria following an offensive by Syrian opposition forces in March, but terror experts warn it is still a threat.

“ISIS very likely will continue to pursue external attacks … against regional and Western adversaries,” the U.S. Director of National Intelligence, Dan Coats, warned in a January report.

Write to Billy Perrigo at billy.perrigo@time.com.