New Zealand security expert Paul Buchanan says it is unlikely the Sri Lanka attacks are retaliation over the Christchurch mosque massacre.

He said given the co-ordinated nature of the attacks and intel warnings which were ignored, it would seem a month is too little time to prepare for such an attack.

SITE Intelligence Group director Rita Katz says IS supporters have boasted on social media Colombo was revenge for the Christchurch mosque massacre.

Katz said IS media channels were "posting rampantly" about the explosions and praying "may Allah accept" the attackers, indicating the group may be planning to claim responsibility.

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"While such a claim may frame the op as revenge for New Zealand, this was likely planned long before," she said.

Security expert Paul Buchanan. Photo / File
Security expert Paul Buchanan. Photo / File

Buchanan said the attack appears to be an attempt to sow sectarian hatred in Sri Lanka, also adding they appear to have been planned prior to the March 15 events.

"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.

"It does show the dangers of tit for tat responses to terrorist attacks of any stripe, as it feeds into the clash of civilisations narrative."

The explosions are the deadliest violence in the country since its civil war ended a decade ago, the Washington Post reported.

The decade that followed has been largely peaceful - few have been killed in any terrorism-related attacks in Sri Lanka.

However, Buddhist nationalism has become its own form of violence, albeit far less deadly than civil war. The attacks appear to be another turn toward religious-based violence.

Blood stains are seen on the wall and on a Jesus Christ statue at the St. Sebastian's Church after blast in Negombo, north of Colombo. Photo / AP
Blood stains are seen on the wall and on a Jesus Christ statue at the St. Sebastian's Church after blast in Negombo, north of Colombo. Photo / AP

The Ministry of Foreign Affairs reports 337 New Zealanders are registered as being in Sri Lanka but there are no reports of injuries or fatalities.

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Meanwhile, any Kiwis with plans to travel to the South Asian country are being told to register with Safe Travel to be alerted to any further risk.

House of Travel commercial director Brent Thomas said travellers should also check in with their travel agent and airline.

Thomas said House of Travel would be working through whether a traveller should cancel or postpone their trip on a case-by-case basis.

The blasts ripped through landmarks in and around Sri Lanka's capital of Colombo, targeting Christians, hotel guests and foreign tourists.

At least 290 people have been named dead and around 500 injured following multiple bombings which exploded on Easter Sunday.

Damage inside of St. Sebastian's Church damaged in blast in Negombo, north of Colombo. Photo / AP
Damage inside of St. Sebastian's Church damaged in blast in Negombo, north of Colombo. Photo / AP

Meanwhile, two Kiwis said they heard "huge explosions" near their Colombo hotel when the first bomb was detonated.

Colleen Dunning from Queenstown and Erin Clark from Auckland had been staying at Galle Face Hotel, along the waterfront from Shangri-La and Grand Cinnamon hotels where explosions occurred.

"We were walking along the waterfront this morning, where families were gathering for a day on the green," Dunning said.

"We heard a huge explosion followed by sirens and then a few minutes later there was another explosion which was very close to our hotel."

They turned back to their hotel, unaware of the scale of the devastation that had happened, and were only informed about 30 minutes later by their tour guide as they made their way out of the city.

They were now down the coast in the city of Galle, and while they were under curfew felt "very safe and have no concerns for our safety".

"We feel for the beautiful people of Sri Lanka," Dunning said.

Prime Minister Jacinda Ardern. Photo / Getty Images
Prime Minister Jacinda Ardern. Photo / Getty Images

Rotorua-based Sri Lankan woman Surangi De Silva Hettiarachchi said one of the explosions happened very close to where she grew up in Colombo.

She watched the news live for hours and said it was awful to see the death toll rising and more explosions going off.

Everyone knows everyone in Sri Lanka, so loads of her friends had lost close ones from the attacks, she said.

Hettiarachchi, who has been living in New Zealand for 14 years, said she was really looking forward to taking her two young girls back home for the first time, but now she felt scared.

She said her community was only just overcoming the losses from 30 years of war and this tragedy highlighted the awful reality that no one was safe in the world.

Elsewhere, New Zealand Prime Minister Jacinda Ardern has described the co-ordinated attacks as "devastating", extending the country's condolences to Sri Lanka.

"New Zealand condemns all acts of terrorism, and our resolve has only been strengthened by the attack on our soil on the 15th of March," Ardern said on Sunday night.

"To see an attack in Sri Lanka while people were in churches and at hotels is devastating.

"New Zealand rejects all forms of extremism and stands for freedom of religion and the right to worship safely. Collectively we must find the will and the answers to end such violence.''