Sri Lankan authorities have confirmed one of the suicide bombers responsible for the Easter Sunday terrorist attacks attended an Australian university.

Key points: One of the suicide bombers completed his post-graduate education in Australia

One of the suicide bombers completed his post-graduate education in Australia The death toll from the deadly attacks was revised up to 359 on Wednesday

The death toll from the deadly attacks was revised up to 359 on Wednesday Investigators say the attacks were retaliation for the Christchurch shootings

Sri Lankan Junior Minister for Defence Ruwan Wijewardene said the bomber completed his post-graduate education in Australia.

It comes after the death toll was revised up to 359 earlier on Wednesday.

"What I can say is this group, some of the suicide bombers, most of them are well educated and come from maybe middle or upper-middle class," Mr Wijewardene said.

"Some of them have studied in various other countries, they hold degrees and they are quite well-educated people.

"We believe that one of the suicide bombers studied in the UK and then maybe later on did his post-graduate in Australia before coming back to settle in Sri Lanka."

Security officers stand in front of St Anthony's Shrine in Colombo, one of the sites targeted in the attacks. ( ABC News: Siobhan Heanue )

Hundreds were killed and 500 injured in a series of coordinated blasts in churches and hotels, in Sri Lanka's deadliest violence since a devastating civil war ended in 2009.

Sri Lanka police say they have confirmed there were nine suicide bombers. Eight have now been identified.

Police spokesman Ruwan Gunasekara said 18 suspects were arrested, raising the total detained to 58.

It is believed Sri Lankan authorities were given multiple warnings about the planned terror attack.

Sri Lankan President Maithripala Sirisena has asked the police chief and defence secretary to resign following the devastating bombings, two sources close to the President told Reuters.

The sources declined to be named due to the sensitivity of the matter amid accusations within the Government of intelligence failures ahead of the attacks.

Mr Wijewardene said authorities were still investigating the possibility of international links and residents were urged to be vigilant.

"I'm not saying that the country is 100 per cent [safe] at the moment … there could be still be a few people out there," he said.

"Right now we are asking people to be vigilant but I think within a couple of days, within the next few days, we will have the situation totally under control."

Clergymen carry coffins for burial during a funeral service for Easter Sunday bomb blast victims at St Sebastian Church in Negombo. ( AP: Gemunu Amarasinghe )

Islamic State claims responsibility as victims are buried

Two domestic Islamist organisations, including National Tawheed Jamaath, are said to be responsible for carrying out the suicide bombings.

Earlier Mr Wijewardene said an initial investigation showed the deadly attacks were carried out in retaliation for last month's Christchurch mosque shootings.

The office of New Zealand's Prime Minister Jacinda Ardern said she was aware of comments linking Sri Lanka's Easter bombings to the mosque attacks in Christchurch, but noted the investigation was "in its early stages".

The FBI and United States military are supporting the Sri Lankan authorities in their investigation, according to the US ambassador to Sri Lanka Alaina Teplitz.

The UK is also reportedly sending a team to help.

"If you look at the scale of the attacks, the level of coordination, the sophistication of them, it's not implausible to think there are foreign linkages," Ms Teplitz said.

"Exploring potential linkages is going to be part of [investigations]," she said.

A national day of mourning in Sri Lanka on Wednesday coincided with mass burials close to the three churches — St Sebastian's in Negombo, St Anthony's Shrine in Colombo and Zion Church in Batticaloa —that were bombed.

As victims were buried, Islamic State (IS) released a statement claiming responsibility for the attacks, but its propaganda channels did not substantiate the claim with evidence.

Black and white ribbons hang in front of St Anthony's Shrine in Colombo as a symbol of mourning. ( ABC News: Siobhan Heanue )

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