A.J. McLean Returns to Rehab—Will It Interfere With Backstreet-New Kids Tour?

Singer back in treatment after a rough few months, has been coping with addiction for many years

This Backstreeter's back—in rehab.

A.J. McLean has returned to an undisclosed rehab center for his third sobriety-seeking stint, a source close to the singer tells E! News.

McLean, who is due on the upcoming reunion tour with The Backstreet Boys and New Kids on the Block, allegedly has been out of control the past few months.

Will his treatment interfere with the concerts?

"A.J. is in ongoing treatment," the source confirms, adding that the tour is "not at all in jeopardy."

The source did not disclose where exactly McLean is receiving treatment but says that he has been struggling with addiction for "many years." He was in rehab twice before, once in 2001 and again in 2002 for both cocaine and alcohol abuse.

McLean, who remains engaged to girlfriend Rochelle Deanna Karidis, wrote hopefully about achieving sobriety on Twitter back on Jan. 9: "I love u all and wish me luck! I start my sober journey tomm!"  The following day he tweeted,  "See u on the other side!"

UPDATE 12:30 p.m. PT: McLean has just released the following statement: "For personal reasons, I've checked myself into a rehab clinic. With 2011 being a busy year for Backstreet, I want to be healthy and perform and sing for people at my very best and happiest. None of this will affect our current tour plans and I look forward to seeing everyone really soon! Thank you for all of your support and respecting my family and friends privacy during this time."