WASHINGTON, Feb. 9 — A Russian intelligence officer working under cover as a diplomat at the United Nations has defected to the United States, say several American officials familiar with the case.
The intelligence officer, identified by the officials as Sergei Tretyakov, defected in October with his wife and other family members and has undergone extensive debriefings by the Federal Bureau of Investigation and the Central Intelligence Agency, officials said.
Mr. Tretyakov's defection was disclosed in late January. At the time, though, American officials described him only as a diplomat and senior aide to Russia's United Nations ambassador, Sergey V. Lavrov.
While Mr. Tretyakov's public title was first secretary in the Russian mission, he was in fact an officer in the S.V.R., Russia's foreign intelligence service, successor to the Soviet-era K.G.B., American officials said.
Several current and former United States officials say defections of Russian intelligence officers have become relatively rare in recent years, partly because American intelligence agencies have become more selective in which Russians they consider to be of interest.
Since Moscow is regarded as less of a threat to the United States now, the market value of Russian spies has declined since the height of the cold war, experts say.
Officials provided few details about Mr. Tretyakov's case, but several current and former officials said his acceptance as a defector when Washington and Moscow were no longer direct adversaries indicated that he had information that the United States considered significant.
Officials refused to say whether Mr. Tretyakov had worked as a spy for the United States while he was still at the United Nations before his defection. If he did so, his direct access to senior Russian officials and secret documents could have increased his value as a defector.
Several experts added that if Mr. Tretyakov was formally accepted into the United States under the laws that govern the C.I.A.'s official defector resettlement program, then he has met fairly high standards in terms of his value to American intelligence.
For instance, it is possible that Mr. Tretyakov would be able to provide American counterintelligence officials with information about some Russian espionage operations under way in the United States.
Mr. Tretyakov's defection has come while Russia has sharply increased the number of intelligence officers it has placed in the United States, officials said.
After being cut nearly in half in the early 1990's after the collapse of the Soviet Union, the ranks of Russian intelligence officers have now returned to nearly their cold war levels, American officials say.
It appears that the United States has not increased its presence in Moscow to a similar degree, officials said, adding that Russia has more intelligence officers in the United States than the United States has in Russia.
For the most part, Russian intelligence officers are based at the Russian Embassy in Washington, the Russian Consulate and United Nations mission in New York, and the Russian Consulate in San Francisco, officials say.
American officials have tried in recent years to persuade Moscow to reduce its intelligence presence here, but without success.
"There was an understanding that there would be a decline in their intelligence presence, but there hasn't been," said one former senior United States official. Some intelligence officials complain that the Clinton administration did not press hard enough or make the matter a priority in United States-Russian relations.
The rise in the numbers of Russian intelligence officers stationed here has also come as the F.B.I. has been shifting some of its focus to new targets, like terrorism, and some officials say the bureau does not have the same number of counterintelligence squads deployed to watch Russians as it did during the cold war.
American intelligence officials say President Vladimir V. Putin, a former K.G.B. officer himself, is in the process of reviving Moscow's security services, and an increasingly assertive Russian presence in the United States seems to fit that pattern.
In December 1999, for instance, the F.B.I. arrested a Russian intelligence officer outside the State Department and uncovered a listening device hidden in a department conference room. American officials also say Russian intelligence has been behind serious attacks on computer systems at the Pentagon and other government sites.
Still, some officials caution that the increased Russian intelligence presence in the United States does not mean that Moscow is winning any new spy war.
In fact, some American officials say it is possible that some Russian intelligence officers have been transferred to this country as a reward and view their assignments here largely as a time to enjoy a pleasant life away from Moscow.