UPDATE 3-Britain's Serious Fraud Office probing AIG unit

(Adds background, additional comment from company)

LONDON Feb 12 (Reuters) - Britain's Serious Fraud Office (SFO) has launched a preliminary inquiry into suspected irregularities at a British unit of American International Group Inc (AIG.N), the SFO said on Thursday.

The probe into dealings at the London office of AIG Financial Products Corp (AIGFP) does not concern the insurance operations of AIG in Britain or elsewhere, the SFO said.

"It is right for us to look into the UK operations of AIG Financial Products Corp to determine if there has been criminal conduct," Richard Alderman, director of the SFO, said in a statement.

"We will use our full range of powers to seek information and to speak to those with an inside knowledge of the company's operations," he added.

The SFO probe widens a multi-agency investigation into how AIGFP valued a derivatives portfolio that took heavy bets on toxic mortgage debt. Also investigating the company are the U.S. Securities and Exchange Commission, the U.S. Department of Justice and Britain's Financial Services Authority (FSA).

An AIG spokesman said the company was cooperating with the SFO investigation and would continue to cooperate with the other investigations.

AIG, once the world's biggest insurer by market value, averted bankruptcy last year after a $152 billion rescue package from the U.S. government.

Its troubles stemmed from losses within the Financial Products group.

AIG is winding down the businesses and portfolios of AIGFP and its subsidiaries. The unit is based in Connecticut and has a large office in London.

"There are approximately 370 employees in AIGFP worldwide who are working on the winding down of the business," AIG said in a statement.

The SFO said it was cooperating with the FSA and U.S. authorities in conducting its own probe. (Reporting by Michael Holden; Additional reporting by Lilla Zuill in New York; Editing by David Holmes and John Wallace)