US citizen Li: Went 'to visit a friend'

"We are very concerned by this... We will press for him to be accorded due process and for his case to be brought to a fair and speedy conclusion resulting in his swift release," a spokesman said.

Mr Li's wife, Liu Yingli, also an academic, said that China's State Security Bureau had telephoned her on Tuesday informing her of the charges.

A professor in business studies at Hong Kong's City University, Mr Li is originally from China but has been a US citizen for the past six years.

China and the United States are already embroiled in a bitter row over the fate of Gao Zhan, a political science researcher at the American University in Washington DC, who was detained on a visit to Beijing in March.

Other detentions

According to Mr Li's wife, her husband crossed the border from Hong Kong to Shenzhen, a city in southern China, on 25 February to see a friend.

He was expected to return to Hong Kong the next day - but he never did.

President Bush has called for the release of Gao

At the time, the authorities refused to make any comment on why he had been detained or where he was being held.

US officials say they have tried to see him, but requests for visits by a UN consular official have been turned down by the Chinese.

China has detained at least five intellectuals with foreign ties, including Ms Gao, who has been detained for spying for unspecified overseas intelligence agencies.

Last month, about 400 leading scholars from around the world signed a petition to Chinese President Jiang Zemin protesting against China's detention of academics.