ISIS supporters celebrated online overnight following the blast in Manchester Arena which claimed the lives of at least 22 people and wounded more than 50 others.
Nobody has yet claimed responsibility for the blast which struck as 21,000 fans were leaving the venue at the end of the Ariana Grande concert.
Two messages, posted four hours before the attack, appeared to predict the atrocity. Twitter suspended dozens of accounts, but other euphoric supporters posted messages across Telegram and other secured messaging systems.
Prime Minister Theresa May will chair a meeting of the emergency Cobra committee at 9am. One message on Twitter posted by a user called Abdul Haqq said: 'It seems that bombs of the British airforce over children of Mosul and Raqqa has just came back to #Manchester.'
Others shared messages encouraging so-called 'lone wolf' attacks as well as videos threatening the west and the United States.
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.At 3am, British police confirmed 19 people were died. By 7am, the death toll had increased to 22. Police said 59 people were injured.
The bomber is believed to have also died in the attack.
The U.S. Department of Homeland Security was monitoring the situation in Manchester closely but said it had no information to indicate a specific credible threat involving music venues in the United States.
British counter-terrorism police have said they are making on average an arrest every day in connection with suspected terrorism.
One user said he hoped ISIS was behind the attack but there has not been any claim of responsibility on any of the group's official social media channels.
One Telegram user said: 'We hope that the perpetrator is one of the soldiers of the caliphate.'
Others posted banners saying 'the beginning is in Brussels and Paris, and in London we form a state,' in reference to previous similar 'lone wolf' attacks in Belgium and France for which the group has claimed responsibility.
British Prime Minister Theresa May said the blast was being treated as a terrorist attack.
If confirmed, it would be the deadliest militant assault on Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005.
U.S. officials drew parallels between the blast and the coordinated attacks in November 2015 by Islamist militants on the Bataclan concert hall and other sites in Paris, which claimed about 130 lives.
