ISIS supporters have been praising the Manchester Arena terror attack in which 22 were killed and another 119 wounded.

Terrifyingly, two tweets appeared to predict the suicide bombing at an Ariana Grande concert attended by thousands of children last night.

Twitter has since confirmed the tweet which read 'Are you forget our threat?' and included a #ManchesterArena hashtag was posted after Britain's deadliest attack since 7/7.

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A Twitter account - which was unverified - posted this four hours before the attack

A sickening video purports to show a jihadist celebrating the Manchester terror attack

Meanwhile a video circulating online today purported to show a masked jihadi celebrating the atrocity, which ISIS has since claimed responsibility for.

The terror group said 'one of the caliphate's soldiers placed bombs among the crowds'.

Twitter shut down 125,000 ISIS-linked accounts last year and said it was hiring more staff to police terrorist propaganda and improve 'algorithms' to find 'hateful material'.

The social media giant refused to comment on last night's concerning tweets 'for privacy and security reasons'.

Its spokesman told MailOnline: 'We always endeavour to support law enforcement as per our guidelines and will continue to support the investigation of the UK authorities as and when required.'

The social media network suspended dozens of accounts but other ISIS supporters continued to post messages on Telegram and other encrypted messaging systems.

Several ISIS-friendly accounts posted the hashtag #ManchesterArena and #ArianaGrande

None of the ISIS fan pages have claimed responsibility for last night's terror attack

The terror group said 'one of the caliphate's soldiers placed bombs among the crowds'

One user, Abdul Haqq, tweeted: 'It seems that bombs of the British Air Force over children of Mosul and Raqqa has just came back to #Manchester.'

TWITTER POLICY ON PRO-ISIS POSTS Twitter announced it had shut down 125,000 accounts with alleged links to ISIS last year. Twitter also said it increased the size of teams policing pro-ISIS posts as well as improving the algorithms to finding 'harmful content'. As of December 2015, the website banned 'hateful content' while its policy on aggressive posts stated: 'You may not make threats of violence or promote violence, including threatening or promoting terrorism.' Twitter did admit the mechanism in identifying pro-Isis tweets was not a 'magic algorithm' capable of pinpointing genuinely terrorist content online. Advertisement

Others shared messages encouraging lone wolf attacks as well as videos threatening the west and the United States.

'This is only the beginning,' one masked man said in a chilling but unverified footage which surfaced today.

The alleged jihadi, sitting in front of an ISIS-style flag, continues: 'The lions of the Islamic State are beginning to attack all the crusaders.'

US security officials said initial signs pointed to a suicide bomber being responsible for the blast.

One counter-terrorism official said: 'In the absence of conclusive evidence, the choice of venue, the timing and the mode of attack all suggest this was terrorism.'

Britain is on its second-highest alert level of 'severe', meaning an attack by militants is considered highly likely.

A primary schoolgirl aged just eight has been named as one of the 22 killed in Manchester last night

Britain is on its second-highest alert level of 'severe', meaning an attack by militants is considered highly likely. Pictured: Concert-goers fleeing Manchester Arena last night

Bloodied concertgoers were pictured being helped by armed police outside the arena after explosions rang out at the gig

Saffie Roussos, 18-year-old Georgina Callander and 26-year-old John Atkinson were among those killed.

Of the injured, 59 were taken to hospital - of which at least 12 were children - and 60 were treated at the scene.

Traumatised families have told how of nuts and bolts tore into young music fans when the blast was detonated in the foyer area of the Manchester Arena moments after a concert by the US popstar ended.

The suicide bomber, who was known to authorities and is said to be British-born, died inside the arena and police carried out a controlled explosion at what is believed to be his home during raids around the city today.

A 23-year-old man was arrested by anti-terror officers in the south of the city as police and security services attempt to work out if the suicide bomber was part of a cell.

Security sources have told MailOnline that initial analysis of the 'sophisticated' device suggests it was made by an expert.

Eight-year-old Saffie Rose Roussos (left), from Preston, and 18-year-old Georgina Callander (right) have been named as victims. It is feared many children are among those killed, as well as parents who had accompanied their youngsters to the concert or were picking them up

Forensics officers emerge after inspecting the horrific scene where a suicide bomber killed 22 people as they left a pop concert at the Manchester arena last night. Police know the identity of the bomber but have not yet named him

This was the scene inside the Manchester Arena last night after the terror attack at the teen concert

Hundreds of people are now desperately trying to track down loved ones who went missing in the aftermath of the attack, with police having to tell some parents their children have died.

Describing the scenes after the blast, one eyewitness told MailOnline she saw a young girl screaming 'I don't know where my sister is'.

Another man told how he cradled a dying woman and saw a girl with her legs blown off.