HARTFORD, Connecticut (Reuters) - A former executive of reinsurer General Re Corp was sentenced to two years probation and fined $10,000 after pleading guilty to helping American International Group Inc cook its books.

US District Judge Christopher Droney, imposing the relatively mild sentence on Richard Napier, said he was taking into account the defendant's extensive cooperation with prosectors.

Napier's testimony helped convict four former General Re executives and one former AIG executive in February 2008, according to prosecutors.

Napier in 2005 pleaded guilty to a conspiracy charge, admitting that he agreed to deals between General Re and AIG that allowed the former insurance heavyweight to artificially inflate its profits. Napier served as senior vice-president of General Re, a reinsurance unit of Warren Buffett's Berkshire Hathaway Inc.

"The words 'I'm sorry' really do not begin to capture my feelings," Napier told the judge before hearing his sentence. "I wish there was a way to turn back the hands of time."

He could have been sentenced to up to five years in prison and fined $250,000.

"Mr. Napier's cooperation with the government ... was of exceptional assistance," Droney said. "His testimony showed the extent of the sham reinsurance conspiracy."

Paul Radvany, a clinical associate professor of law at Fordham Law School in New York and a former prosecutor, said: "It's helpful to the system for people to cooperate, and for them to cooperate they need an incentive, which is usually a lighter sentence."

He added that the judge may have weighed the fact that Napier did not directly profit from the fraud in making his sentencing decision.

The government had alleged that several AIG and General Re executives engineered a sham reinsurance transaction in 2000 that inflated AIG's loss reserves by $500 million, without actually transferring risk.

Napier had been an account representative for the AIG account.

Former AIG vice-president Christian Milton was sentenced to four years in prison for his role in the scheme.

Former General Re chief executive Ronald Ferguson was sentenced to two years in prison, former General Re chief financial officer Elizabeth Monrad to 18 months, and former General Re senior vice-president Christopher Garand and Assistant General Counsel Robert Graham to one year each.