The terrorist tracking group SITE first reported ISIS' statement praising the "soldier" behind the attack.
Teenage screams filled the arena just after the explosion Monday night, which also killed the attacker and injured dozens.
The Prime Minister said the official threat level remains at severe - meaning an attack is highly likely - but this would be assessed throughout the day.
"I won't call them monsters because they would like that term".
At least 22 people, including children, were killed and about 60 others were wounded after explosives were detonated at the end of the pop star's show at the Manchester Arena, police said.
While police believed the suspect was acting alone, investigation continued to establish if he was part of a network.
Britain's third biggest city was hit in 1996 by a massive auto bomb planted at a shopping centre by the Irish Liberation Army (IRA) which wounded more than 200 people.
Contrary to many other Western nations, most British police officers across the country do not carry firearms.
Messages of support poured in from around the world, including from President Trump.
Puri added: "We should be really careful not to give too much credence to an organization that effectively operates by inspiring people it may have never had any contact with".
Charlotte Campbell has not heard from her 15-year-old daughter Olivia since the attack.
The people of Manchester have shown extraordinary courage, compassion and resilience in the face of terror.
Nearly every PC hit by WannaCry ransomware runs Windows 7
The tool does not need any internet connection, keeping you safe from any risk of being infected while scanning. A security researcher noticed that WannaCry was pinging a specific domain, one that was not registered.
"I am both shocked and heartbroken by the horrific explosion at the Manchester Arena in England", Cuomo said in a statement Monday.
"Ariana is OK. We are further investigating what happened", he said.
It is the worst terrorist strike on British soil since 2005, when Islamist extremists bombed the London subway and a bus, killing 54 people.
"We are all very sad about the tragic events last night; we can not take out of our minds and our hearts the victims and their families", the Portuguese said.
We stand in absolute solidarity with the people of the United Kingdom. She tweeted several hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words".
"People were running and screaming". Family members and friends posted tributes to him on social media.
But none have been able to trace Saffie despite calling the police crisis phone number and local hotels and mustering points for the injured.
It requires event organisers to notify the police a month beforehand if they expect more than 5,000 people to attend. "Some of them looked dead". She said authorities were trying to determine if he had an accomplice.
He later found them, uninjured. "As we came outside to Victoria Station, there were just people all over the floor covered in blood".
The 21,000-seat venue had been sold out. The attack took place near one of the exits of the facility, in a public space.
A spokeswoman said: "The safety of our customers is of the utmost importance".