New Edition star Bobby Brown has bowed out of the R&B band's ongoing U.S. tour due to medical reasons.

The veteran Cool It Now hitmakers kicked off their latest trek in Connecticut on 26 June (14) and have been performing for fans in cities like Boston, Massachusetts, Philadelphia, Pennsylvania and Brooklyn, New York in the past two weeks.

However, audience members have noticed Brown failing to keep up with his bandmates during a number of synchronised dance routines, with the singer appearing to struggle to catch his breath as his fellow New Edition members continue with the show.

He was even filmed briefly walking off stage during their gig in Southaven, Mississippi last week (02Jul14), and now the singer's representative has revealed the reason for his lack of stamina.

The spokesperson tells Brown recently underwent "an intrusive medical procedure", which has left him exhausted and fatigued, prompting him to quit the tour.

Brown, 45, has since been placed under doctors' care as he continues to recover from the mystery ailment and he has reached out to fans to say sorry.

The note reads: "I apologize to the fans of New Edition. I really wanted to be with my brothers, but presently I am unable to give the fans what they typically expect from New Edition."

The tour is due to continue in Oklahoma on Friday night (11Jul14) and wraps up in Los Angeles on 3 August (14).