At least 19 people have been killed and 50 injured in an explosion which is treated as a "terrorist incident", at a pop concert by US star Ariana Grande in the Manchester city in the UK, police said today.

"Police were called to reports of an explosion at Manchester Arena. So far 19 people have been confirmed dead with around 50 injured," Greater Manchester Police said in a statement.

"This is currently being treated as a terrorist incident until police know otherwise," the statement said.

Eyewitnesses reported hearing a loud "bang" from inside the venue, which was playing host to a concert by American pop star Ariana Grande. Video footage from the scene showed bloodied victims being helped by emergency services.

Grande is safe, her spokesperson said.

"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," an eyewitness said.

The blasts reportedly went off at the end of the concert near the box office area of the venue.

"Some people were screaming they'd seen blood but other people were saying it was balloons busting or a speaker had been popped," said another eyewitness.

The UK's Network Rail said train lines out of Manchester Victoria station, which is close to the concert venue, were blocked.