* Thorough financial investigation under way - official
* Ex-finance minister was arrested, released last year
* Five other people also under investigation - official
By Munkhchimeg Davaasharav and Barbara Lewis
ULAANBAATAR/LONDON, Jan 29 (Reuters) - Mongolia is working with overseas investigators to look into claims of corruption at its giant Oyu Tolgoi copper mine, the country's anti-graft body said on Tuesday, after the re-arrest of a former minister suspected of "abuse of power".
Bayartsogt Sangajav was first arrested last April and released in June in a probe into 2009 negotiations over the development of the mine, then owned by Canada's Ivanhoe Mines and now managed by Anglo-Australian miner Rio Tinto RIO.AX RIO.L . Bayartsogt, a former finance minister, was among the signatories to the investment agreement, approved by Mongolia's parliament. Tumur, a spokeswoman with the country's Independent Authority Against Corruption (IAAC), told Reuters it was now conducting a thorough financial investigation into the allegations that would be international in scope. She did not identify with which other countries the IAAC was collaborating.
The mine, run by Rio Tinto since 2010, is central to the firm's push to reduce its reliance on iron ore sales. But the miner has faced a series of wrangles with the Mongolian government over tax payments and cost overruns. plans will make Oyu Tolgoi, located near the southern border with China, one of the world's biggest copper mines early in the next decade.
"An investigation is ongoing into suspected abuse of power - that former finance minister Bayartsogt held shares in a foreign company before the Oyu Tolgoi investment agreement," the head of investigations at the IAAC, Idertsog Erdene, told a news conference on Friday.
Idertsog said authorities were looking into allegations that the ex-minister, one of six people now being investigated in relation to the case, "gained a large amount of money by selling the shares when the prices went up after the signing agreement". He didn't identify the foreign company.
Bayartsogt was re-arrested on Jan. 16, according to Mongolian media reports.
The former minister, who has previously denied all charges, is now in custody, according to his assistant, and unavailable for comment. His assistant and lawyer both declined to comment.
The 2009 deal gave 34 percent of the Oyu Tolgoi copper-gold project to the government of Mongolia, with the remainder held by Ivanhoe, now known as Turquoise Hill Resources TRQ.TO . Turquoise Hill is now 51 percent-owned by Rio Tinto.
Turquoise Hill and Rio declined requests to comment.
The Swiss Office of the Attorney General (OAG) is also conducting a separate criminal investigation into a seized bank account that court documents indicated was used to transfer $10 million to Bayartsogt in September 2008. an emailed statement, the OAG said it could not discuss further details with the criminal investigation still ongoing.