Supporters of Islamist militant group Isis are celebrating online after a suicide bomber hit a music concert by American pop star Ariana Grande in Manchester, UK, killing at least 22 people and wounding dozens of others on 22 May.

No terror group has yet taken responsibility for the attack, but police are treating it as a terror attack. Police also said children are among the dead.

Isis supporters reportedly took to social media platforms such as Twitter and Telegram to hail the incident and call for more "lone wolf" attacks in the West. Some users claimed that the attack was revenge against the US-led coalition's air strikes in Iraq and Syria.

"It seems that bombs of the British airforce over children of Mosul and Raqqa has just came back to #Manchester," a user named Abdul Haqq wrote on Twitter, referring to the major Iraqi and Syrian cities, respectively, that are currently the anti-Isis battlegrounds. The UK is also a vital member of the coalition and is actively fighting terrorists both in the region and at home.

Another Isis supporter wrote on a channel affiliated to the militant group and hosted by messaging network Telegram: "We hope that the perpetrator is one of the soldiers of the caliphate."

Some users posted banners saying "the beginning is in Brussels and Paris, and in London we form a state", referring to the previous terror attacks in Belgium and France, in which alleged Isis supporters drove heavy vehicles into crowds and killed dozens of people, or bombed public places, Reuters reported.

"It's now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack, an attack that targeted some of the youngest people in our country with cold calculation," said UK Prime Minister Theresa May in a statement outside 10 Downing Street.

"This was among the worst terrorist incidents we have ever experienced in the United Kingdom. Although it is not the first time that Manchester has suffered in this way, it is the worst attack the city has experienced and the worse ever to hit the north of England."

She added: "Today let us remember those who died, and let us celebrate those who helped, safe in the knowledge that the terrorists will never win and our values, our country and our way of life, will always prevail."

People from the music world, including Grande, have reacted to the shocking incident. "From the bottom of my heart, I am so so sorry. I don't have words," Grande tweeted following the attack. She is reported to be safe, but in deep shock after the incident.