LONDON — The attack on a Manchester pop concert that killed 22 people was "likely" the work of more than one person, British interior minister Amber Rudd said on Wednesday (May 24).

"It was a devastating occasion, it was more sophisticated than some of the attacks we've seen before, and it seems likely - possible - that he wasn't doing this on his own," she told BBC radio.

Ms Rudd confirmed that bomber Salman Abedi, a British man of Libyan heritage who died in the explosion on Monday night, had been on the radar of the security services.

"We do know that he was known up to a point to the intelligence services," she told Sky News.

Clarifying this on the BBC she said: "The security services will know a lot of people. It doesn't mean that they're expected to arrest everybody they know.

"But it is somebody that they had known."

She said she had "complete confidence" in the security services.

The minister said she was "not surprised at all" that the attack had been claimed by the Islamic State group, but said there was no information yet to confirm the extremist organisation's active direction.

Britain's national terror threat level was raised late Tuesday to "critical", meaning another attack may be imminent, following the attack on the concert of US singer Ariana Grande.

London police said Wednesday they would be calling in the army to help guard key landmarks, including Buckingham Palace, the Houses of Parliament and foreign embassies.

UK 'IRRITATED' BY US LEAKS 

Ms Rudd added in her BBC radio interview that it was "irritating" that details about the Manchester concert bombing had been leaked to US media before being released in Britain, saying she had spoken to US authorities.

"The British police have been very clear that they want to control the flow of information in order to protect operational integrity... the element of surprise," said Ms Rudd. 

"So it is irritating if it gets released from other sources and I have been very clear with our friends that that should not happen again." 

Asked if the US authorities had compromised the investigation, she said: "I wouldn't go that far.

"But I can say that they are perfectly clear about the situation and that it shouldn't happen again."

Abedi's name first surfaced in US media reports on Tuesday, based on briefings given to US officials by their counterparts in London. AFP