Mr Yang, 40, has been in detention for 15 months

Yang Jianli, who runs a Boston-based foundation advocating democratic change in China, was detained in April last year during a visit to the country.

Mr Yang was expected to plead not guilty at Monday's closed-door trial in Beijing, which ended after three hours without a verdict being announced.

His wife, Christina Fu, said she was very worried about the outcome.

"Chances of the court meting out a not guilty (verdict) are small," she told Reuters news agency.

There is uncertainty over what sentence Mr Yang might face, but a conviction for espionage can carry the death penalty in China.

The case has come to court despite intense international pressure for Mr Yang's release.

Last week the US Senate condemned his continued detention, and warned that cases like Mr Yang's could harm China-US relations.

Espionage claims

Mr Yang is accused of acting as an agent for the Nationalist Party, the former ruling party of Taiwan, according to his wife's lawyer Jared Genser.

Mr Genser said the charges appeared to relate to a series of grants given by Mr Yang's organisation, the Foundation for China in the 21st Century, to groups within China.

Mr Yang's supporters have said these donations were entirely unrelated to espionage. One donation, they said, was for an agricultural scientist's research into papaya trees.

Mr Yang left China in 1989 after the suppression of the pro-democracy protests in Tiananmen Square.

He was banned from re-entering the country, but did so last year - allegedly using a friend's passport - in order to investigate labour unrest in northern China.

As well as the charges of espionage, he also faces the less serious charge of entering the country illegally.

In recent years, China has detained a number of its citizens with links to the US, on charges of spying for Taiwan. Most were convicted and then expelled.

Taiwan has been ruled separately from China since 1949, but Beijing claims the island is part of its territory.

Both sides are believed to actively spy on each other.