(Reuters) -- British police have said they are treating the blast at the arena in Manchester at the end of a concert by U.S. singer Ariana Grande as a "terrorist incident". More than 50 people were wounded.

Twitter accounts affiliated to ISIS have used hashtags referring to the blast to post celebratory messages, with some users encouraging similar attacks elsewhere.

"It seems that bombs of the British airforce over children of Mosul and Raqqa has just came back to #Manchester," one user named Abdul Haqq said on Twitter, in reference to the Iraqi and Syrian cities held by the militants where a U.S.-led coalition is conducting air strikes.

Supporters posted messages encouraging each other to carry out "lone wolf" attacks in the West and shared ISIS videos threatening the United States and Europe.