A prominent Beijing scholar who recently fled to the United States warned Thursday that China was sending "spies" to US universities and urged caution on academic cooperation.

Xia Yeliang is one of the original signatories of Charter 08, a bold petition for reform whose Nobel Prize-winning lead author Liu Xiaobo is in prison. Xia, an economist, was fired in October from Peking University, often considered China's most prestigious university.

In his first public event since moving to the United States last month, Xia said he was mindful of the 1950s McCarthyist era, when smears of alleged communist sympathies sullied the reputations of Americans in government, entertainment and academia.

But Xia, who has been a visiting scholar at several US universities, said he was aware of "real spies" sent by China to the United States to carry out surveillance in the guise of academic exchanges.

"Every year there are some visiting scholars -- among them, I can definitely say some of the people are actually spies. They don't do any research," Xia said at the Cato Institute, a libertarian-oriented Washington think tank which has made him a visiting fellow.

Xia also urged US universities -- which have been increasingly opening campuses for eager students in China and Gulf Arab monarchies -- to make sure they "keep up some basic values like freedom of speech."

"If Hitler is here and he tried to make some cooperation with Western universities ... would you also like to accept that?" Xia asked.

"Some people would say, 'Oh, you cannot compare with that.' But there's some aspects (in which) it's quite similar," he said.

Xia said he hoped US universities would keep contacts with Chinese counterparts and encourage enrollment of Chinese students, but called for a clear-eyed look at the other side's motivations.

"You have to keep that in mind -- maybe those people just want to borrow your good names, and ruin your names," he said.

Peking University said that Xia was fired for poor teaching. Xia rejected the charges, saying he had received positive reviews during his 13 years at Peking University and daring administrators to fire other professors over their teaching abilities.

One of China's most famous dissidents, blind self-taught lawyer Chen Guangcheng, last year accused New York University of dismissing him due to political pressure as it opened a campus in Shanghai.

The private Manhattan university denied the allegations, saying it only intended to keep Chen for a year and questioning why it would take him in at all if it was currying favor with China.