The US Embassy in Sri Lanka warned Thursday that places of worship could be targeted for militant attacks over the coming weekend, as police searched for more suspects in the Islamic State-claimed Easter suicide bombings that killed over 350 people.

The stark warning came as Sri Lankan Prime Minister Ranil Wickremesinghe told The Associated Press that he believed militants, likely with access to explosives, remained on the loose in the island nation off the southern tip of India.

"We have rounded up a lot of suspects but there are still active people on the run," Wickremesinghe said in an interview.

"They may be having explosives with them, so we have to find them." Police meanwhile issued a public appeal for information about three women and one man suspected of involvement in the Easter bombings.

Across Colombo and in Negombo, the seaside city north of the capital where a bombing killed 110 people, residents and office workers stayed indoors and closed up shops Thursday, leaving both cities unusually quiet.

The attacks Easter Sunday, mainly at churches and hotels, killed at least 359 people and wounded 500 more, the government said Wednesday. Most were Sri Lankan but the Foreign Ministry confirmed on Thursday that 40 foreigners died.

The Islamic State group has claimed responsibility for the attacks. Authorities remain unsure of the group's involvement, though they are investigating whether foreign militants advised, funded or guided the local bombers.

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