Joining foreign terror outfit not against Sri Lankan law: PM

However, the government has blamed a local Islamist extremist group, National Tawheed Jamath, for the bombings.

"We knew they went to Syria...But in our country, to go abroad and return or to take part in a foreign armed uprising isn't an offense," Wickremesinghe told Sky News.

"We have no laws which enable us to take into custody people who join foreign terrorist-groups," he added.

"We can take those who are, who belong to terrorist groups operating in Sri Lanka," the prime minister was quoted as saying.

Facing public criticism for not acting against the Islamist extremist groups in the island nation, Prime Minister Wickremesinghe said some of the suspected attackers responsible for the Easter bombings were being monitored by the country's intelligence services.

However, the authorities did not have "sufficient" evidence to place the suspected attackers in custody prior to the attacks, Wickremesinghe said.

Top officials have acknowledged that Sri Lanka received intelligence about possible terror strikes ahead of the attacks, but both President Maithripala Sirisena and Prime Minister Wickremesinghe have said that they did not receive any information of that sort.

Wickremesinghe told CNN that he did not rule out the possibility of more attacks in the country.

He said authorities were now focusing on "sleepers"- terrorists who could activate another round of attacks.

"Police and security forces are rounding up those involved, but they're also rounding up the sleepers, those used on second and third rounds (of attacks)," he said.

"The danger has come down drastically, (but) we do have to pick up some more sleepers," Wickremesinghe said.

Sri Lankan police have released photos of six suspects, including three women, wanted for their involvement in the attacks and sought information regarding them from the public.

Authorities earlier said nine bombers, believed to be members of NTJ, carried out the blasts.