Sergei Tretyakov

Sergei Tretyakov was a former Russian spy who defected to the US after running espionage operations from the United Nations.

Tretyakov, who died on 13 June in Florida aged 53, defected in 2000 and later claimed that his agents helped the Russian government steal nearly $500m from the UN's oil-for-food program in Iraq. He was 53, according to a Social Security death record.

"Sergei was called 'the most important spy for the US since the collapse of the Soviet Union' by an FBI official in my book," said Pete Earley, who published a book about Tretyakov in 2008. "Unfortunately, because much of what he said is still being used by US counterintelligence officers, it will be years before the true extent of his contribution can be made public – if ever."

Tretyakov was born on 5 October 1956 in Moscow. He joined the KGB and rose quickly to become the second-in-command of its UN office in New York between 1995 and 2000. His defection in 2000 was very significant as Russia's spies in the US would have come under his purview, according to Peter Earnest, director of the International Spy Museum in Washington, who spent more than 30 years in the CIA.

For up to a decade following his defection, the FBI kept watch over 10 Russian agents as they tried to blend into American suburbia. They were arrested last week and swapped last Friday in Vienna for four people convicted in Russia of spying for the US and Britain.

In a 2008 interview promoting Earley's book, Tretyakov said his agents helped the Russian government skim hundreds of millions of dollars from the oil-for-food programme before the fall of Saddam Hussein.

Tretyakov called his defection "the major failure of Russian intelligence in the United States" and warned that Russia, despite the end of the Cold War, harboured bad intentions toward the US. He said he found it immoral to continue helping the Russian government.

"Finally in my life, when I defected, I did something good in my life," he said, because I want to help the US."