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A sick video has surfaced online showing an ISIS jihadist saying "this is only the beginning" after the Manchester 'terror attack' that killed more than 20 .

In the unverified footage, a man wearing a mask and sitting in front of a black flag making the ominous warning.

The man then holds up a sign bearing the words "Manchester 2017-05-22", a reference to yesterday's date.

Hours later ISIS claimed responsibility for the attack via the Reuters news agency and the killer was named as Salman Abedi.

In the video the masked man says: "In the name of Allah the merciful, the grateful, this is only the beginning.

"The lions of the Islamic State are beginning to attack all the crusaders. Allahu akbar. Allahu akbar."

(Image: Internet Unknown) (Image: Internet Unknown)

He then holds up his sign bearing the date of the attack.

This video appeared on social media and the Mirror can not verify the source of the authenticity.

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But just hours later an ISIS statement said: "One of the soldiers of the Caliphate was able to place an explosive device within a gathering of the Crusaders in the city of Manchester."

The atrocity in Manchester left 22 dead and 59 injured after a suicide bomber detonated an improvised explosive device .

The victims named so far include eight-year-old Saffie Rose Roussos and 18-year-old Georgina Callander.

(Image: SWNS) (Image: Rex Features) (Image: SWNS)

Hundreds of people fled the packed arena screaming after hearing 'loud bangs' at the end of an Ariana Grande concert last night.

Pictures of children and teens who attended the concert were being circulated on social media in a desperate bid to track them down.

A mum waiting at the arena for her daughters earlier described how she saw a man with "risen parts" in his clothing, whom she believes was the bomber.

(Image: PA)

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Announcing the devastating news, he said: "What I can confirm is that there are children among the deceased.

"This has been the most horrific incident we have had to face in Greater Manchester and one we all hoped we would never see.

(Image: PA)

"Families and many young people were out to enjoy a concert at the Manchester Arena and have lost their lives.

"Our thoughts are with those 22 victims that we now know have died, the 59 people who have been injured and their loved ones.

"We continue to do all we can to support them."

(Image: PA)

Prime Minister Theresa May said in a statement in Downing Street: "We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish, but as an opportunity for carnage.

"But we can continue to resolve to thwart such attacks in future. To take on and defeat the ideology that often fuels this violence.

"And if there turn out to be others responsible for this attack, to seek them out and bring them to justice.

"The police and security services believe the attack was carried out by one man. But they now need to know whether he was acting alone or as part of a wider group.

"It will take some time to establish these facts and the investigation will continue.

"The police and security services will be given all the resources they need to complete that task."