Permira, Bain Capital and a team of CVC Capital and Blackstone have made offers in the final round of bidding for Citigroup Inc's Japanese telemarketer Bellsystem24, seven people with direct knowledge of the deal said.

The deal could fetch more than $1 billion, making it the largest private equity transaction in Japan since March last year.

The people spoke on condition of anonymity because the bidding process is not public.

No one at Citigroup in Tokyo could be reached for comment.

Citigroup put Bellsystem24 up for sale as part of its global efforts to raise cash and bolster its capital. The U.S. bank also sold its retail broker Nikko Cordial Securities and asset management company Nikko Asset Management.

The sale of Bellsystem24 initially drew strong interest from a number of private equity firms including Kohlberg Kravis Roberts & Co KKR.UL, which teamed up with Itochu Corp before dropping out of the bidding.

Bellsystem24 operates call centers and competes against Moshi Moshi Hotline Inc and Transcosmos Inc in Japan.

Estimated at around $1 billion the deal would be the biggest transaction involving foreign private equity since March 2008, when Permira bought agrichemical company Arysta LifeScience Corp and Advantage Partners bought Tokyo Star Bank.

Both deals were valued at about $2.2 billion.

(Additional reporting by Emi Emoto; editing by Edwina Gibbs and Karen Foster)