GENEVA/LONDON (Reuters) - Swiss police on Thursday searched the Geneva offices of Onyx Financial Advisors, a company providing management services for BSGR, the mining arm of Israeli billionaire Beny Steinmetz's business empire, an Onyx spokesman said.

Onyx said it had provided information to Swiss authorities following a request made by the government of Guinea, which is reviewing BSGR's concessions in the West African country and its right to mine half of Simandou, one of the world's largest untapped iron ore deposits.

Guinea has accused BSGR of paying bribes to obtain its concession under a previous government.

BSGR has repeatedly denied wrongdoing and in turn accused Guinea's government of trying to use the process to confiscate its rights to Simandou.

A security guard at the property, near Geneva airport, said about 20 police officers arrived in the early morning, brought up documents from the basement and stayed for several hours.

"Onyx has absolutely nothing to hide. We are co-operating fully with the authorities but are unable to comment further at this time," the spokesman said.

A spokesman for the Geneva police declined to comment. BSGR had no comment.

As part of a U.S. corruption probe involving Guinea, FBI agents in April arrested BSGR agent Frederic Cilins on charges of obstructing a criminal investigation, tampering with a witness and destroying records. Two men have also been arrested in Guinea.

Onyx is a company which BSGR has described as wholly separate and fully independent, although it provides administrative and management services for BSGR. Onyx's chief executive is also a director on the board of BSGR.

(Reporting by Clara Ferreira-Marques in London and Emma Farge in Geneva; Editing by Erica Billingham)