"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.
President Maithripala Sirisena, who is also defence and law and order minister, said he will carry out a complete reorganisation of the security forces and the police under his control.
Meanwhile, Sri Lankans held a mass burials on Tuesday near the St Sebastian Church in Negombo. White flags and ribbons have been put up at shops, roadsides and other places in the country to mourn the loss. The country has marked the national day of morning on Tuesday with flags at half mast and a minute of silence observed at 8:30 am.
Nearly 60 people have been detained since the Sunday blasts.