The militant Islamic State has claimed responsibility for coordinated bombings in Sri Lanka which killed 321 people and injured about 500 others.

Two of the suicide bombers blew themselves up at the luxury Shangri-La Hotel on Colombo's seafront, said Ariyananda Welianga, a senior official at the government's forensic division. The others targeted three churches and two other hotels.

A fourth hotel and a house in a suburb of the capital Colombo were also hit, but it was not immediately clear how those attacks were carried out.

Most of the attacks came during Easter services and when hotel guests were sitting down for breakfast buffets.

Police had received a tip-off of a possible attack on churches by a little-known domestic Islamist group some 10 days ago, according to a document seen by Reuters.

The intelligence report, dated April 11, said a foreign intelligence agency had warned authorities of possible attacks on churches by the leader of the group, the National Thawheed Jama'ut. It was not immediately clear what action, if any, was taken on the tip-off.

International anti-terrorism experts said even if a local group had carried out the attacks, it was likely that militant groups Al Qaeda or Islamic State were involved, given the level of sophistication.

“Those that carried out the attack that targeted members of the US-led coalition and Christians in Sri Lanka the day before yesterday are Islamic State group fighters,” said a statement released by IS propaganda agency Amaq said on Tuesday.

The group did not give evidence for its claim.