Usher and Shakira sign up as mentors for The Voice US

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Usher and Shakira have signed up as mentors for the next series of The Voice in America.

Christina Aguilera and Cee Lo Green are taking a break from the NBC show, which is currently airing its third season.

The fourth series will also feature original coaches Adam Levine from Maroon 5 and Blake Shelton.

Cee Lo Green is developing new music and a sitcom based on his life for NBC, while Christina Aguilera is going on tour to support her new album.

She released a statement saying: "As I have expressed since day one of the show... I am a singer and performer first and I am so excited to get back to that love."

She confirmed that she would be eager to return to The Voice in the future as did Cee Lo Green who said he will be back for series five.

The Forget You singer also encouraged Usher and Shakira to make themselves at home in the coaches' chairs.

NBC entertainment chief, Bob Greenblatt, said that the network was glad to support Aguilera and Green by allowing them to take time away from the program.

Shakira is a Colombian singer-songwriter and dancer whose song Waka Waka (This Time for Africa) was chosen as the official song of the 2010 FIFA World Cup.

More recently she has been working on a new album.

Usher is one of the biggest selling artists in the world, whose recent hits include OMG and Yeah! He is also credited with signing Justin Bieber.

He also recently appeared on Oprah's Next Chapter to discuss the custody battle over his two young sons and the death of his stepson Kile Glover after a jet ski accident.