This church in Colombo was targeted in the Easter Sunday blasts in Sri Lanka (AFP)

COLOMBO: Prime Minister Ranil Wickremesinghe said the government knew that Sri Lankan nationals who joined the Islamic State had returned, but they could not be arrested as joining a foreign terrorist organisation is not against the law in the island nation.

But the Easter Sunday serial blasts that killed over 250 people have forced a rethink among the country's political top brass.

Wickremesinghe on Friday said the country needs new laws to deal with threats posed by local terror outfits linked to ISIS, the group which has claimed responsibility for the Easter bombings that left 253 people dead and several hundreds injured.

"The definition on aiding terrorism is very narrow. Therefore, the laws are not strong to deal with a situation like this," Wickremesinghe said in a televised address.

"We have to widen the scope of these laws to counter global terrorism. Not only they (the terrorists) should be arrested, their assets also need to be confiscated," he added.

The Islamic State group has claimed responsibility for the Easter terror attacks on three Catholic churches and three luxury hotels but the government has blamed a local Islamist extremist group, National Tawheed Jamath (NTJ), for the bombings.

Earlier in an interview to Sky News, the prime minister said the government knew that Sri Lankan nationals who joined the Islamic State had returned, but they could not be arrested as joining a foreign terrorist organisation is not against the law in the island nation.

"We knew they went to Syria...But in our country, to go abroad and return or to take part in a foreign armed uprising is not an offence here,” Wickremesinghe said.

"We have no laws which enable us to take into custody people who join foreign terrorist groups. We can take those who are, who belong to terrorist groups operating in Sri Lanka," he told the news channel.

Admitting failure of the Lankan intelligence on the attacks, Wickremesinghe said although he had not been fed with prior information, he as the prime minister takes full responsibility for the failure.

Wickremesinghe said some of the suspected attackers responsible for the Easter bombings were being monitored by the country's intelligence services.

But authorities did not have "sufficient" evidence to place the suspected attackers in custody prior to the attacks, he 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 Mnister Wickremesinghe have said that they did not receive the information.

Wickremesinghe told CNN that he did not rule out the possibility of more attacks in the country. He said authorities were now focussing 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, which we will do in the next few days."

He said although the attacks were carried out in the island nation, they might have been remotely controlled at another country.

The planning of the attack and training might have been in another country, he said, adding, "Without destroying this network we will fail in our task to eliminate global terrorism."

