One of the suicide bombers involved in the Easter Sunday attacks across Sri Lanka studied in the UK and Australia, a Sri Lankan minister has said.

“We believe that one of the suicide bombers studied in the UK and later did his postgraduate [studies] in Australia before coming back and settling in Sri Lanka,” said Ruwan Wijewardene, the state minister for defence, at a media briefing on Wednesday afternoon.

Wijewardene confirmed that many of the bombers had international connections, having lived or studied abroad.

“This group of suicide bombers, most of them are well-educated and come from middle or upper-middle class, so they are financially quite independent and their families are quite stable financially, that is a worrying factor in this,” he said. “Some of them have I think studied in various other countries, they hold degrees, LLMs [law degrees], they’re quite well-educated people.”

A spokesperson for the Australian Department of Home Affairs confirmed on Wednesday that one of the alleged bombers had previously held a visa for Australia, but would not comment further on the matter.

On Wednesday, the death toll from the attacks that tore apart several churches and hotels in cities around Sri Lanka rose to 359, including 39 foreign nationals, with 500 injured.

Eighteen suspects were arrested overnight, bringing the number detained to 58, police spokesman Ruwan Gunasekara said. But Sri Lanka’s prime minister, Ranil Wickremesinghe, warned that several suspects armed with explosives were still on the loose. Up to nine people directly linked to the attack could still be at large, according to sources involved in the investigation.

Wijewardene said the investigation was continuing and authorities expected to make further arrests in the coming days.

“We can firmly say in the next couple of days our security agencies will have the situation of this country firmly under control,” he said.

On Tuesday, Sri Lanka’s prime minister, Ranil Wickremesinghe, warned that several suspects armed with explosives were still loose and acknowledged there had been a prior warning about potential attacks.

A Sri Lankan security official told the Guardian that warnings had been shared by Indian authorities on 4 April and 9 April. Another warning may have been issued by India on Saturday night, an official told Reuters.

Wijewardene responded to questions about the involvement of Islamic State in the attacks, saying the bombings may have been funded and ideologically-inspired by the group.

Islamic State has claimed responsibility for the Easter bombings through the group’s Amaq news agency and published a statement saying its “fighters” were responsible and listing the names of the suicide bombers, who were also shown in a video swearing allegiance.

Wijewardene also stood by his comments that the attacks were carried out as a reprisal for the attacks on two mosques in Christchurch, saying that this was the assessment of Sri Lanka’s security services, despite analysis from experts that an attack of this magnitude would have taken months to plan.

On Wednesday, New Zealand prime minister Jacinda Ardern said her government had received neither official communication from the Sri Lankan government nor any intelligence regarding the claim. “We have nothing at this point to corroborate what has been said,” Ardern said.

“According to an assessment done by the intelligence services, they believe it was a reprisal,” said Wijewardene. “The Christchurch incident motivated these guys to carry out this blast on Easter Sunday… According to the assessment by our intelligence sources, this is what they believe.”