US pop star Ariana Grande has said she is "broken" following the deadly terror attack on her concert at Manchester Arena, adding: "From the bottom of my heart, I am so so sorry."

The tweet came around five hours following an explosion that killed at least 22 people, including children, and injured more than 50 others. Police believe a lone suicide bomber was behind the atrocity.

Earlier, a representative for Grande said in the immediate aftermath she was "Okay", Variety reported.

Grande’s music label, Universal Media Group said: "We are deeply saddened to learn of tonight's devastating event in Manchester. Our thoughts and prayers are with all those affected by this tragedy."

Witnesses reported hearing two loud bangs coming from near the arena's bars at about 10.35 p.m.

"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," concertgoer Majid Khan, 22, told the Press Association. "It was one bang, and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit."