At least 19 people were killed in a blast at a concert in Manchester on Monday where U.S. singer Ariana Grande had been performing, reports Reuters.

British police said the incident, in which at least 50 people were injured, was being treated as a terrorist incident.

Britain is on its second-highest alert level of "severe" meaning an attack by militants is considered highly likely.

A spokesman for Ariana Grande, 23, said the singer was "okay".

Ariana's friends and colleagues from the industry were shocked by the attack.