A former top US diplomat has pleaded guilty to not disclosing a relationship with a Taiwanese intelligence officer.
Donald Keyser was seen meeting Isabelle Cheng by FBI agents
Donald Keyser, 62, also pleaded guilty to illegally removing classified documents from the US State Department where he was employed until 2004.
He faces up to eight years in jail when sentenced in February. He denies having a sexual relationship with the woman.
Washington is a major backer of Taiwan, but in 1979 it switched diplomatic ties from the island to China.
China considers Taiwan to be a renegade province.
Mr Keyser - a former Number Two official in the State Department's section handling East Asian affairs - admitted having a personal relationship with Isabelle Cheng, 34.
During a hearing at a court in Virginia, he made an apology, saying he wanted to keep it a secret from his wife.
"I understand that my personal relationship with her (Ms Cheng) might have made me vulnerable to attempted exploitation or pressure," he said.
FBI agents monitored Mr Keyser in 2004 and found that on a number of occasions he had met Ms Cheng and her boss at Taiwan's National Intelligence Bureau in the Washington area, officials said.
They said that thousands of State Department files - some marked top secret - had been found during a subsequent search at Mr Keyser's home.
"In all, Keyser had over 3,600 documents in either hard copy or electronic form, " the US Justice Department said in a statement.
Released court documents also showed that he failed to disclose that he had made a four-day visit to Taiwan in 2003.
Mr Keyser had also served at US embassies in Beijing and Tokyo during his career spanning three decades.