The Islamic State group on Tuesday claimed a series of bombings that killed more than 320 people in Sri Lanka, and released a photo and video of the men it said were responsible.

The massive casualty toll would make the Easter Sunday attacks the deadliest overseas operation claimed by IS since the group proclaimed its caliphate in mid-2014.

"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," IS propaganda agency Amaq said in a statement.

In a later statement, the group gave the noms de guerre of seven people it said were behind the "blessed attack" that targeted Christians during their "blasphemous holiday".

The Islamic State group published a picture of eight men it said were behind the attacks in Sri Lank

Amaq also released a photo of eight men it said were behind the blasts.

Seven of them had their faces covered and three of them held knives.

The one man who displayed his bearded face also appeared to be carrying an assault rifle.

Amaq later released a video of the eight fighters pledging allegiance to IS supremo Abu Bakr al-Baghdadi in Arabic, a black IS flag hanging in the background.

The authenticity of the image and video could not be independently verified and the reason for the discrepancy in the reported number of attackers was not immediately clear.