One of the attackers who carried out the devastating suicide bombings in Sri Lanka on Easter Sunday had studied in the UK and Australia, the country’s defence minister has said.

Islamic State has claimed responsibility for the bombings, believed to be the most lethal ever conducted by the group.

‘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 at a media briefing on Wednesday, reports t theguardian.com.

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.’

British counter-terrorism investigators said they were unaware Sri Lanka was going to announce publicly that one of the suspected attackers had British links. A Metropolitan police spokesperson said: ‘We do not discuss who we may or may not be investigating as part of counter-terrorism investigations.’

On Wednesday, the death toll from the attacks on three churches and three luxury hotels in and around the capital, Colombo, rose to 359, with 500 injured.

Eighteen suspects were arrested overnight, bringing the total number detained to 58, said a police spokesman, Ruwan Gunasekara.

Despite the scale of the security operation, Sri Lanka’s prime minister, Ranil Wickremesinghe, warned that several suspects armed with explosives had yet to be found.

Wickremesinghe said there were more explosives and militants ‘out there’, and confirmed reports there had been a failed attack against a fourth major hotel, and that the Indian embassy was also a possible target.

Up to nine people directly linked to the attack could still be at large, sources involved in the investigation said.

The defence minister 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,’ said Wijewardene.

One of the nine bombers was a woman who killed herself and two children when police closed in on a safe house. She is believed to have been the wife of one of two brothers who attacked two hotels.

Among those detained was the Sri Lankan landlord who had provided rental accommodation to some of the attackers. His sister told the Guardian her brother had done nothing wrong. ‘They are saying the one who blasted [on Sunday] has come to his house, but this is not true at all,’ she said. ‘He is a very innocent guy.’

The developments came as Sri Lanka’s president called for a massive security overhaul. Maithripala Sirisena called for the resignation of the most senior defence and police officials for their failure to act on repeated warnings transmitted by Indian and other intelligence services in the weeks and days before the attacks.

Sirisena has repeatedly said he had not been made aware of the warnings.

Alaina Teplitz, the US ambassador in Sri Lanka, told reporters that ‘clearly there was some failure in the system’ but said the US had no prior knowledge of a threat before the attacks, the worst violence in the south Asian island nation since its civil war ended a decade ago.

Teplitz called that breakdown in communication at the top levels of government ‘incredibly tragic’.

The warnings – made two weeks before the attacks, with possibly a further communication about a potential threat just hours before the blasts – appears to have been based on Indian agencies’ interrogations of Isis sympathisers arrested late last year in the southern state of Tamil Nadu.

The detainees are reported to have revealed the existence of a plot to launch mass casualty attacks led by Zahran Hashim, a radical Islamist cleric based in the east of Sri Lanka, who may have been among the suicide bombers who attacked on Sunday.

Hashim could be seen in a video released by Isis on Tuesday following its claim of responsibility. Dressed in a black tunic and headscarf and carrying a rifle, the cleric is seen leading a group of men, said to be Sunday’s attackers, in a pledge of allegiance to the leader of Isis, Abu Bakr al-Baghdadi.

Western and south Asian officials said that Hashim had started a group of extremists in a small village in eastern part of the island and attracted disaffected young men. However the middle-aged cleric travelled overseas about two years ago. Security agencies followed Hashim’s online extremist activism closely and were concerned that he could develop links to either al-Qaida or Isis.

Experts said that although Isis had made false claims of responsibility in the past, it appeared likely that its involvement in the suicide bombings on Sunday would be confirmed by the investigations.

The group’s Amaq news agency also published a statement saying its ‘fighters’ were responsible and listed the names of the suicide bombers.

Among those arrested in Tuesday’s roundup were six Pakistani refugees, including two women and two children.

The inspector at Katana police station, MA Senavirathne, said CCTV cameras and telephone evidence had shown that the alleged bomber of St Sebastian’s church in Negombo may have visited the refugees at their home.

Funerals of those killed in the blasts continued in Negombo and Colombo on Wednesday, as a social media ban and state of emergency remained in place across the country.