Police made arrests in the UK and Libya as the investigation into a suicide bomber who killed 22 people at a Manchester concert venue packed with children focused on tracking down a network of accomplices who authorities fear could strike again.

The father of the suspected bomber, identified as Salman Abedi, 22, told the Reuters news agency in the Libyan capital on Wednesday that he had last spoken to his son some five days ago by phone and "everything was normal".

Ramadan Abedi, who was detained by a Tripoli counterterrorism force during the interview, said his son Salman had told his family that he was heading on pilgrimage to Mecca.

"I spoke to him about five days ago ... there was nothing wrong, everything was normal," Abedi said.

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The suspect blew himself up on Monday night at the Manchester Arena indoor venue at the end of a concert by US pop singer Ariana Grande attended by thousands of children and teenagers.

The attack was claimed by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.

Abedi also said he was sure Salman had not been a member of ISIL.

"Salman doesn't belong to any organisation," he said. "The family is a bit confused because Salman doesn't have this ideology, he doesn't hold these beliefs. We condemn these terrorist acts on civilians, innocent people."

Police in Tripoli also arrested a brother of Abedi. A spokesman for the local counterterrorism force said younger brother Hashem Abedi was arrested on suspicion of links with ISIL and was suspected of planning to carry out an attack in the Libyan capital.

A man arrested on Tuesday, one day after the attack at the Manchester Arena, was reported by British and US media to be Abedi's other brother.

Manchester police, meanwhile, made several new arrests.

"We currently have eight people in custody in relation to Monday's attack," Ian Hopkins, chief constable of Greater Manchester Police, said in a statement on Thursday.

"The arrests have taken place in Manchester, Wigan and Nuneaton, and we are now carrying out associated searches in relation to those arrests at a number of addresses," he added.

Earlier, interior minister Amber Rudd said the bomber had recently returned from Libya. Her French counterpart Gerard Collomb said he had links with ISIL and had probably visited Syria as well.

Rudd also scolded US officials for leaking details about the investigation into the Manchester attack before British authorities were ready to go public.

The bomb used in the attack appeared to contain carefully packed shrapnel and have a powerful, high-velocity charge, according to leaked photographs from the investigation published by The New York Times.

"We are furious. This is completely unacceptable," a government ministry source said of the images "leaked from inside the US system".

Hopkins, the Manchester police chief, said the leaks had "caused much distress for families that are already suffering terribly with their loss".

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The Manchester bombing raised concern across Europe.

Cities including Paris, Nice, Brussels, St Petersburg, Berlin and London have suffered attacks in the past two years.

The 22 victims in Manchester included an eight-year-old girl, several teenage girls, a 28-year-old man and a Polish couple who had come to collect their daughters.

Britain's official threat level was raised to "critical", the highest level, late on Tuesday, meaning an attack was expected imminently.

The Manchester bombing was the deadliest attack in Britain since July 2005, when four British Muslim suicide bombers killed 52 people in coordinated attacks on London's transport network.

Rudd said up to 3,800 soldiers could be deployed on Britain's streets, taking on guard duties to free up police to focus on patrols and investigation. An initial deployment of 984 had been ordered, first in London and then elsewhere.

Soldiers were seen at the Houses of Parliament, Theresa May's Downing Street residence and at the London police headquarters at New Scotland Yard.