Justin Bieber is sick whilst performing at a US gig

Video caption, Amateur video shows Justin Bieber being sick on stage at his concert in Glendale, Arizona

Justin Bieber has apologised to fans after vomiting while on stage in the US on Saturday night.

He was performing at a sell-out concert in Glendale, Arizona as part of his Believe tour of America.

An online video shows the singer being sick on two occasions and leaving the stage after each incident.

It is reported that his fans, known as Beliebers, sang some of his songs as they waited for the 18-year-old singer to come back.

He managed to complete the show, even returning to do an encore.

Later Justin Bieber went on Twitter to let fans know he was starting to feel better.

"Great show. Getting better for tomorrow's show!!!! And... Milk was a bad choice! Lol"

Bieber's 45-city Believe tour continues in Los Angeles on Tuesday and Wednesday.

He is due to come to the UK in February as part of his European tour.