China on Friday defended its overseas Confucius Institutes, one day after a US congressman called for a probe into the language and cultural centres over concerns that they violate academic freedom.

The row over the Beijing-backed centres -- which number nearly 100 in the US, mostly on the campuses of publicly-funded state universities -- comes after a number of US, Canadian and other schools publicly cut ties with them.

Chinese foreign ministry spokeswoman Hua Chunying described Confucius Institutes as "a bridge linking China with the rest of the world" and maintained that all activities at the centres were "open and transparent".

"The Chinese side provides assistance in terms of teachers and textbooks at the request of the American side," Hua said at a regular briefing. "It has never interfered in academic freedom."

"We hope that all countries can make joint efforts to discard prejudice and build the bridge of friendship much stronger," she added.

On Thursday, Congressman Chris Smith, who sits on the US House of Representatives foreign affairs committee, voiced concern that the institutes are muting criticism of Chinese authorities.

"Is American higher education for sale?" said the New Jersey Republican, a longtime critic of China's Communist authorities who also co-chairs the Congressional-Executive Committee on China.

"And if so, are US colleges and universities undermining the principle of academic freedom -- and in the process their credibility -- in exchange for China's education dollars?"

In recent months, several colleges and universities have broken ties with Confucius Institutes over concerns about their impact on free expression.

The University of Chicago, which has had a Confucius Institute since 2009, announced in September that it was suspending talks on a renewal of the centre's contract. More than 100 professors had signed a petition urging the school to shut the centre.

Days later Pennsylvania State University announced that it would close its Confucius Institute by the end of the year. Canada's McMaster University, the University of Sherbrooke and the Toronto District School Board have made similar decisions.

Proponents of the centres argue that they give increasingly cash-strapped schools a way to provide students with valuable Chinese language and cultural instruction.

Critics contend that the institutes present a whitewashed view of Chinese history and that administrators who accept Chinese funding do so with an implicit understanding that discussion of certain sensitive topics -- such as Taiwan, Tibet or the Tiananmen Square crackdown -- is off-limits.

Those opposed to the centres also point to instances of discrimination in their hiring practices. McMaster last year faced a complaint from a former teacher who was an adherent of the Falun Gong spiritual movement, which is banned in China.

The teacher said that her contract barred her from joining "illegal organisations" such as Falun Gong, according to a report in Canada's Globe and Mail.