The information passed on related to Taiwan's new air defence system
A US defence department analyst has admitted giving classified information about military communication systems to a businessman working for China.
Gregg Bergersen, 51, pleaded guilty to a charge of conspiracy to disclose national defence information "to persons not entitled to receive it".
Mr Bergersen faces up to 10 years in prison when he is sentenced on 20 June.
Correspondents say his admission comes amid growing concern in Washington about the activities of Chinese spies.
Four others were arrested in separate case last month for allegedly passing secret details about the space shuttle and other US aerospace programmes to China.
Air defence system
Mr Bergersen, a weapons systems policy analyst at the US Defence Security Co-operation Agency (DSCA), was arrested last month with joint Taiwanese-US national Tai Shen Kuo and Yu Xin Kang, a Chinese national living in the US.
The FBI said Mr Bergersen had received thousands of dollars for passing on classified information to Mr Kuo, a New Orleans-based furniture salesman who has also been accused of turning over that information to the Chinese government.
The US government said that Mr Bergesen thought Mr Kuo was closely affiliated with Taiwan's ministry of defence and was unaware he was in contact with Chinese officials.
Mr Bergesen's lawyer, Mark Cummings, said that there had been no explicit exchange of money for information. For instance, Mr Bergesen had won $3,000 from Mr Kuo in cash in a poker game in Las Vegas in April 2007, he said.
"In hindsight, he understands that the money was given to him in anticipation that he would provide documents," Mr Cummings added.
Ms Kang ferried the information between Mr Kuo and Chinese officials, the FBI alleged.
Mr Kuo, 58, and Ms Kang, 33, face a more serious charge of "conspiracy to disclose national defence information to a foreign government". They face up to life in prison if convicted.
The information passed on related to Taiwan's new Po Sheng air defence system. Taiwanese officials said that some damage had been caused by the disclosures, but that they had not compromised key technology.
The Chinese government has dismissed the espionage accusations as groundless and accused the US of "Cold War thinking".