The British government has raised the country's terror threat level from "severe" to "critical" in the wake of the bombing of a concert hall in Manchester.
Police have also named the suspected suicide bomber who killed 22 people in an attack on the Manchester Arena as Salman Abedi.
The Islamic State militant group claimed responsibility for Monday night's that also injured 59 others at a concert by U.S. pop singer Ariana Grande in the city in northern England.
The 22-year-old was Manchester born and from a family of Libyan origin, the BBC reported. The police said the priority is to establish whether Abedi has worked alone or not.
British Prime Minister Theresa May said the city had fallen victim to a "callous terrorist attack" and police believe they know the attacker's identity.
A statement from the prime minister said the country's Joint Terrorism Analysis Centre "has now concluded...that the threat level should be increased, for the time being, from severe to critical. This means that their assessment is not only that an attack remains highly likely, but that a further attack may be imminent."
The government announced that members of the armed forces are to be deployed around the United Kingdom to support armed police officers.
Manchester Arena, one of the biggest indoor arenas in Europe with capacity for 21,000 people, said on its website the incident "took place outside the venue in a public space" as people were leaving the show.
Local police arrested a 23-year-old man in connection with the blast, but his relation to the attacker was not known.
Meanwhile, local media has named three of the people who died in the blast, including an 8-year-old girl.
A number of people are still unaccounted for.
The incident follows a terrorist attack involving a vehicle near parliament in London on March 22 that left five people dead and more than 50 injured.
Manchester Arena, which opened in 1995, attracts more than 1 million visitors annually.
A vigil was held in the city on Tuesday in the aftermath of the attack.
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