A former high-ranking State Department official who is one of the nation's leading experts on China passed documents to Taiwanese intelligence agents and was charged yesterday with concealing a trip to Taiwan, court papers say.

Donald W. Keyser, who was elevated to principal deputy assistant secretary for East Asian and Pacific Affairs this year, made the trip last year, according to an FBI affidavit filed in U.S. District Court in Alexandria. Keyser, 61, who advised Secretary of State Colin L. Powell on China issues, met with one of the agents in Taipei last September during an official trip to China and Japan, the affidavit says.

Tailed by the FBI in recent weeks, Keyser and two Taiwanese agents conducted a series of covert meetings around Washington. At a meeting July 31 at the Potowmack Landing restaurant, the affidavit says, Keyser handed the Taiwanese two envelopes "that appeared to bear U.S. government printing.''

On Sept. 4 at the same Alexandria restaurant, on the Potomac River with a view of downtown Washington, FBI agents saw Keyser pass a document captioned "discussion topics,'' the affidavit says. FBI agents stopped the three men outside the restaurant and took the six-page document, described in the affidavit as something "derived from material to which Keyser had access as a result of his employment with the Department of State."

The court documents do not say that Keyser accepted money and do not otherwise ascribe a motive. Neither Keyser nor his attorney returned phone calls yesterday.

Keyser told the FBI that the document he gave the two Taiwanese agents contained "talking points" that he often would prepare for his meetings with the two agents, according to the affidavit. He said that his trip to Taiwan had been for sightseeing and that he had not notified anyone about it, including his family. His wife is a CIA officer.

State Department spokesman Richard A. Boucher said the department is cooperating with the FBI, but he declined to comment further. The affidavit does not describe the documents Keyser allegedly handed over as classified, and it is unclear whether any damage could have been done to national security.

Keyser is charged with concealing the trip to Taiwan by lying in May on State Department forms for security clearance that required him to disclose foreign travel.

News of Keyser's arrest stunned some in diplomatic circles, in which he is highly regarded as a China analyst. Keyser, a Foreign Service officer for three decades, speaks fluent Mandarin and is knowledgeable about the former Soviet Union. He has served in high-ranking positions in the U.S. embassies in Beijing and Tokyo, and was deputy assistant secretary of state for East Asian and Pacific Affairs when he allegedly made the trip to Taiwan.

Keyser retired in July as the No. 2 person in the State Department's East Asia bureau, but he is still assigned to the department's Foreign Service Institute in Arlington.

"He is an absolutely superb specialist on China and a fine Foreign Service officer. I've never had the slightest reason to question his loyalty to the United States,'' said J. Stapleton Roy, a three-time U.S. ambassador who was Keyser's boss when Keyser was deputy director of the State Department's Bureau of Intelligence and Research.

Roy quit his job in protest in 2000 after then-Secretary of State Madeleine K. Albright suspended Keyser for 30 days and reassigned him because of lax security stemming from a missing top-secret laptop computer. Keyser was one of at least six State Department employees disciplined over the loss of the computer, which contained thousands of pages of information about weapons proliferation issues and was never found. Roy said yesterday that Keyser had nothing to do with the computer's disappearance.

This is the second recent instance of a federal official being implicated in passing documents to countries friendly with the United States. The FBI is investigating whether Lawrence A. Franklin, a Pentagon policy analyst, provided a draft presidential directive on Iran to the American Israel Public Affairs Committee, and whether that committee passed the information to Israel, law enforcement sources have said. No charges have been filed.

The United States has a longstanding "one China" policy, under which it maintains diplomatic relations only with China, not with Taiwan. But Chinese officials recently have expressed frustration over the Bush administration's willingness to sell arms to Taiwan. China and Taiwan are adversaries, with China insisting that Taiwan reunite with the mainland.

According to court documents, Keyser traveled to China on official State Department business about Aug. 31, 2003. He then went to Tokyo on official business, but while in Tokyo took a three-day "side trip" to Taiwan, the documents say.

While in Taiwan, Keyser met with someone referred to in court documents as "Foreign Person One." He is described as a Taiwanese intelligence agent stationed at the Taipei Economic Cultural and Representative Office on Wisconsin Avenue in the District.

Court documents say James A. Kelly, assistant secretary of state for East Asian and Pacific Affairs and Keyser's superior at the time, told the FBI that Keyser was not permitted to travel to Taiwan on official business because the United States and Taiwan don't have diplomatic relations and that he would have vetoed such a trip.

Experts were surprised that Keyser would travel to Taiwan right after visiting China.

"The whole idea that he could take a trip like this that was not authorized while he was deputy assistant secretary is ludicrous to me. People in that position don't just move around anonymously,'' said a former high-ranking State Department official who specialized in Chinese affairs and who spoke on condition of anonymity because of the case's sensitivity.

Keyser appeared in federal court in Alexandria yesterday and later was scheduled to be released on $500,000 bond.

Staff writer Walter Pincus contributed to this report.

Former official Donald Keyser is charged with concealing trip.