The White House yesterday strongly denied that a previously unknown high-ranking KGB officer had defected from the Soviet Union to the West last year, a denial that added a new touch of mystery to Washington's latest spy story.

U.S. News and World Report said in its Monday issue that the defector had been a major general in the KGB, the Soviet secret police, and had brought "extremely valuable" information to the West. Its account said this defector, labeled "the fifth man," was higher ranking than the four spies that the Central Intelligence Agency acknowledges fled to the West last year.

In an unusually flat denial, White House spokesman Larry Speakes said yesterday, "That story is not correct." When Speakes was then asked which "aspect" of the story he was denying, he replied, "The whole thing."

Reporters asked additional questions, and Speakes persisted in his denial, saying, "This KGB defector that was described in U.S. News and World Report is not so."

Behind the scenes, the denials were even stronger. While the CIA officially followed its policy of declining to comment on such stories, administration officials and congressional sources said that the intelligence agency had denied the account in strong terms.

Responding to the denials, James C. Killpatrick, senior editor, administration, at U.S. News, issued a statement defending the story.

"We reported the story from multiple sources over a period of several weeks," Killpatrick said. "Based on the reputations and numbers of these sources, we believe our story is correct. We were warned by more than one of our sources that we could expect denials from the CIA and possibly other government agencies."

In response to further questions about the authenticity of the story, Killpatrick said, "We stand by our story."

Sources within the Reagan administration and at the magazine said the story had been in progress for at least four weeks and its accuracy had been questioned throughout that period. But editors at the magazine said they were satisfied the story is correct.

The story caused a furor in Washington yesterday. White House officials, aware of its sensitivity, questioned CIA officials about its accuracy. They were told, said one official, that the story was "baseless."

As the story was being reported, according to one well-informed source, it triggered specific inquiries from one congressional oversight committee to CIA officials about whether there was some "surprise" defector whose existence had not been disclosed in private briefings on Capitol Hill.

The uniform CIA response, according to this source, was that no such defector existed.

Several officials said there has been a steady stream of low-level defectors from Soviet-bloc nations. Some of these officials suggested that it was possible that U.S. News had magnified the importance of one of these defections.

The New York Times reported in its Sunday editions that congressional sources had confirmed the magazine story but added few details. It quoted a congressional source as saying that the defector is providing information that "is much more important" than that provided by Vitaly S. Yurchenko, the celebrated defector who re-defected to the Soviets last year.

The U.S. News account said the "fifth man" defector was "feeding secrets" to the United States before he fled the Soviet Union. The magazine described him as a middle-aged Russian and "professional with technical expertise" who often traveled to Soviet-bloc countries.

The account said he was smuggled out of East Germany in late April or early May, hidden to prevent press leaks during the Geneva summit in November and, "because of the Yurchenko fiasco, settled in the Midwest."