LOS ANGELES (Reuters) - R&B star Usher and Colombian singer Shakira will replace Christina Aguilera and Cee Lo Green for the fourth season next year of NBC's hit singing contest "The Voice," the network said on Monday.

The changes in the celebrity judge line-up will allow Aguilera and Green to work on new projects. Aguilera is releasing a new album in November and going on tour, while Green is developing a scripted comedy series for NBC based on his life, NBC entertainment chairman Robert Greenblatt said in a statement.

Judge-mentors Adam Levine, frontman of pop group Maroon 5, and country singer Blake Shelton will stay in place for season four, which starts in the spring of 2013.

"Doing two back-to-back cycles of this show requires a tremendous amount of time, and we are happy to accommodate the commitments made by Christina and Cee Lo," Greenblatt said.

"The participants on our show benefit from our coaches' expertise and their continued success in the music business allows for significant new contributions to the contestants when they return," he added.

The line-up change for "The Voice" follows the announcement on Sunday that rapper Nicki Minaj and country star Keith Urban will join rival Fox TV singing contest "American Idol" when it returns in January 2013. Mariah Carey has also signed up for "Idol."

Usher, best known for his 2004 album "Confessions," is one of the world's best-selling artists with global sales of 65 million record and seven Grammy awards.

Shakira burst onto the international pop scene with her 2006 single "Hips Don't Lie."

"The Voice" has been a bright spot in struggling NBC's schedule since its 2011 debut, and moved to a twice a year cycle this month. Its third season opener last week notably drew more U.S. viewers than the revamped version of Fox's "The X Factor," with Britney Spears making her debut as a judge.

Fox in a unit of News Corp and NBC is majority-owned by Comcast.