House Democratic Whip Steny H Hoyer said there must be no doubt that those who instigated or facilitated this attack will face justice.
The bombing appeared meant to inflict the maximum possible damage on young concertgoers - many of them in their early teens - who were making their way out of the Manchester Arena.
Confirming the details of the attack, Chief Constable of Greater Manchester Police Ian Hopkins said the investigation to identify the perpetrators behind the deadly attack is underway, until which the attack is being treated as a terrorist incident.
Youthful fans who were at USA pop star Ariana Grande's concert in Manchester and parents waiting for their children outside described scenes of panic after an explosion ripped through the foyer on Monday night. "I've called all the places, the hotels where people said that children have been taken and I've called the police".
Britain's terrorist threat level has been set at "severe" in recent years indicating an attack is highly likely.
However, the cause of the reported blast is still not known. "The priority is to establish whether he was acting alone or as part of a network".
Others said they were "covered in people's blood, skin".
"In the absence of conclusive evidence, the choice of venue, the timing and the mode of attack all suggest this was terrorism", a United States counterterrorism official who also spoke on condition of anonymity said.
The local hospital, Wythenshawe, said it was dealing with "mass casualties".
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It is not the first such attack on a European concert venue. Many parents and teens later gathered at a nearby Holiday Inn that was established as a meeting point.
"This apparent act of terrorism, targeting a concert attended by thousands of teenagers and young people, is an inexplicable and abhorrent assault on our universal values as human beings", Cuomo said in a statement. "Some of them looked dead".
Photos claiming to show Ariane Grande bruised and bloodied after the concert were really photos from 2015, during a break from shooting a video.
"We heard the last song go and then suddenly there was a massive flash and then a bang and smoke", he said. They will not change us.
A controlled explosion was carried out by police at the Cathedral Gardens area near Manchester Arena shortly after 1.30am. A loud bang was heard minutes later. Police said there are 19 confirmed deaths.
Nineteen people died in a suspected terror attack at a pop concert in the British city of Manchester late Monday.
Meanwhile, relatives of concert-goers caught up in the terror attack at the Manchester Arena are desperately searching for missing loved-ones using social media.
"I'm 10 mins away from the arena and if anyone needs food drinks or charge your phone or a sofa or anything get in touch #roomformanchester", a tweet user posted.
Nineteen people have been confirmed dead and more than 50 injured, according to the Greater Manchester Police. The two assailants, who were convicted of murder, said they were acting to avenge the killing of Muslims by British soldiers.
Campaigning has been suspended in Britain's national election after a deadly explosion at Manchester Arena.