Police say the death toll in the Easter attacks in Sri Lanka has risen to 359 and more suspects have been arrested.

Police spokesman Ruwan Gunasekara also said Wednesday morning that 18 suspects were arrested overnight, raising the total detained to 58.

The prime minister warned on Tuesday that several suspects armed with explosives were still at large.

Another top government official said the suicide bombings at the churches, hotels and other sites were carried out by Islamic fundamentalists in apparent retaliation for the New Zealand mosque massacre last month.

Ten days before the Easter Sunday bombings in Sri Lanka, a top police official warned about an "alleged plan attack" from a radical Islamic extremist.

A member of the National Thowheet Jama'ath group was found using social media to spread "hate speech against non-Muslims" following the Christchurch mosque shootings.

The New York Times translated a memo in which the high ranking police officer said it had been observed the man was posting regularly online.

The document was titled "Information of an alleged plan attack" and was signed by Deputy Inspector General of Police, Priyalal Dassanayake.

"A person known as Milhan, maintains a social media account under the name Mohammed Milhan and interacts with the social media accounts of Zaharan," the memo said.

"It has been observed that he has been regularly updating accounts with hate speech against non-Muslims since the March 15, 2019, attacks on a Muslim mosque."

No direct link is mentioned between the Christchurch attacks and the Sri Lankan bombings.

The New York Timers reports top government officials said the warning never reached them and no action was taken against National Thowheet Jama'ath.

Meanwhile, security experts in New Zealand have told the Herald and Newstalk ZB it is unlikely there is a link between both terrorist attacks.

Dr Paul Buchanan, director of 36th Parallel Assessments, said given the coordinated nature of the attacks it was unlikely they were planned so quickly.

"Christchurch seems to be a convenient justification for something that was being planned before March 15 and has more to do with ethnic-religious conflict in Sri Lanka," he said.

Chris Kumeroa, also a New Zealand terrorism and security expert, agreed with Buchanan's sentiments when talking with Tim Dower on Newstalk ZB.

Kumeroa said attacks like those on Easter Sunday generally take long periods of time to plan, fund, and a lot of reconnaissance would have been undertaken.

"I don't see any evidence to support that at all. I mean it's probably coincidental," he said.

"They would have picked timings and the Easter period to conduct these types of attacks ... it would have had to have been under the radar for a long period of time.

"And bearing in mind to that terrorists cells operate in isolation so they have the bomb making cell separate to those conducting the activity."

Yesterday, Islamic State claimed responsibility for the attacks which have killed over 300 and wounded around 500 others in Sri Lanka.

State Minister of Defence Ruwan Wijewardene said initial investigations showed the attacks were carried out in retaliation against the Christchurch attacks.

Last month, the New York Times reported Abu Hassan al-Muhajir, the spokesman for Isis, called for retaliation after 50 Muslims were killed in the Christchurch attacks.

After the threat, Dr Buchanan said with Isis fighters returning to their home countries after defeat in Syria "the threat is real".

However, the level of threat depended on how New Zealand responded as a nation and rallied around its Muslim community, he said.

"Defeat in Syria is sending Isis fighters back to their home countries to resume decentralised, small-unit and lone-wolf operations against soft targets.

"Massacre gives them a recruiting tool and incentive [revenge]. The threat is real.

"But New Zealand may be safe if the non-Muslim population rallies around the Muslim community.

"How we respond as a nation will determine the level of threat."