Three men were arrested today in Manchester in an investigation into the suicide bomb attack which killed 22 people at a music concert in the northern English city on Monday night, police in the city said.

"Three police warrants were executed in south Manchester in connection to the ongoing investigation," a spokeswoman for Greater Manchester Police said.

Britain has increased its security threat level to "critical" from "severe" following the suicide attack, Prime Minister Theresa May said earlier.

The Islamic State terror group has claimed responsibility for the bombing which killed 22 victims, including an eight-year-old girl, in the worst terrorist incident to hit Britain since the July 7 attacks.

A lone bomber detonated a homemade device packed with nuts and bolts in the foyer of the Manchester Arena as thousands of young people were leaving a concert by US pop star Ariana Grande.

The suspected attacker was named as Salman Abedi, aged 22. Two US officials who have been in contact with British authorities said he was believed to have travelled to Manchester from London by train.