A prominent Beijing scholar who recently fled to the United States has warned that China was sending "spies" to American universities, and urged US institutions to tread carefully on academic co-operation. Xia Yeliang is one of the original signatories of Charter 08, a petition for reform whose Nobel Prize-winning lead author Liu Xiaobo is in prison. Xia, an economist, was fired in October from Peking University. In his first public event since moving to the US last month, Xia said on Thursday he was mindful of the 1950s McCarthy era, when smears of alleged communist sympathies hit 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 Beijing to the US to carry out surveillance under the guise of academic exchange. "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 Washington think tank which has made him a visiting fellow. Xia urged US universities - which have been opening campuses for students from China and Gulf Arab monarchies - to ensure they "keep up some basic values like freedom of speech". "If Hitler were here and he tried to make some co-operation with Western universities … would you also like to accept that?" he said. "Some people would say, 'Oh, you cannot compare with that.' But there's some aspects [in which] it's quite similar." Xia said he hoped US universities would keep contacts with Chinese counterparts and encourage enrolment of Chinese students, but called for a clear-eyed look at their motivations. Peking University said Xia was fired for poor teaching, charges which Xia rejected.