Islamic State supporter tweets Manchester with 'ISIS flag', prior to blast

At least 19 people were killed and 50 injured at Ariana Grande's concert

According to unconfirmed reports, an unknown twitter account of a possible Islamic State sympathiser sported a tweet about the Manchester Arena in England, hours before the explosion rocked pop star Ariana Grande's concert.

"#Islamic State #manchesterarena #UK #British ARE YOU FORGET OUR THREAT? THIS IS THE JUST TERROR (sic)" read the tweet @ owys663. However, the account has been suspended but several users re-tweeted. Here are some of the tweets:

Meanwhile, an unverified video has also appeared on the social media purportedly showing a man claiming "loyals" of ISIS were behind alleged 'nail bomb' attack inside Manchester Arena.

At least 19 people were killed and 50 injured after what is being investigated as a suspected suicide bombing of a crowded pop concert in Manchester, the most deadly attack in Britain in a decade.

Last, England saw such a deadly terror attack was in 2005 when on July 7, which killed 52 people after terrorists carried out a series of coordinated suicide bomb attacks in central London targeting civilians using the public transport system during the rush hour.

Also, in 2009, the Manchester police had thwarted a major terror bid to attack Manchester's Arndale shopping centre on the busy Easter bank holiday weekend.

With up to 90,000 shoppers in or near the shopping centre at the time, police believe an attack would have killed hundreds and maimed thousands.

A student identified as Abid Naseer, 29, who plotted the mass suicide bomb attack was jailed for 40 years in 2015.