BEIJING (AP) — Dozens of scholars in Australia, the U.S., Britain and Hong Kong are urging Beijing to let a Sydney-based academic return home, warning that continued barring of his departure is raising concerns about the risks of conducting research in China.

Organizers of an open letter to President Xi Jinping and Premier Li Keqiang say more than 80 scholars have signed it so far, and it will be sent next week after the number of signatories is expected to exceed 100.

It calls for authorities to allow Feng Chongyi, an associate professor at the University of Technology Sydney, to return to Australia after he was preventing from doing so last week on suspicion of endangering national security.

Feng is a Chinese citizen and permanent resident of Australia, according to his lawyer. His case was cited earlier this week by politicians in Australia whose opposition over human rights concerns prompted the government to abandon efforts to seek ratification of an extradition treaty with China.

Chinese Foreign Ministry spokesman Lu Kang said Thursday that Chinese law enforcement authorities “while performing duties of safeguarding national security” prevented Feng going abroad.

“Feng Chongyi, as a Chinese citizen, is obliged to cooperate with the relevant investigation by Chinese authorities,” Lu told reporters.

Feng had been wrapping up a three-week trip researching human rights lawyers. Since July 2015, authorities have questioned or detained hundreds of activists and independent legal professionals as part of a crackdown on civil society under President Xi. Some of the lawyers have been labeled threats to national security.

Rights groups and Western governments, including the U.S., have urged China to release the activists and lawyers. Activists say the crackdown is aimed at silencing critics of the ruling Communist Party in violation of the Chinese Constitution.

The open letter to China’s top two leaders states that the signatories are “disturbed that a fellow researcher, who has dedicated himself to promote the understanding of and interest in China, has been prevented from returning to his home and workplace for no reason other than his conscientious work as a China Studies scholar.

“Such actions make it difficult for the rest of us to be confident in the research environment in China today, and do not contribute positively to the continued construction of open and productive higher education collaboration between China and the rest of the world.”

Organizers plan to deliver it to the Chinese Embassy in Canberra next week.

One of them, Kevin Carrico, said the academics were worried about the safety of Feng, who has been a “kind, very welcoming” member of the Australian academic community who organized talks and debates on issues in Chinese society and China-Australia relations.

Carrico, a lecturer in Chinese studies at Macquarie University in Sydney, said the scholars wanted to make a “clear statement that this is a matter of real concern for anybody who does research in China, anybody who works in academic exchange with China.

“This type of a development of course doesn’t contribute positively to the goal of mutual exchange between China and the outside world, which I know Professor Feng is an advocate of and that we all advocate.”

While detentions of overseas-based academics such as Feng are rare, scores of foreign scholars in the China field have been denied visas to conduct research within China, apparently due to political considerations.

Border officials at an airport in the southern city of Guangzhou refused to let Feng catch flights home on Friday and Saturday last week, as Premier Li was making an official visit to Australia.

Feng’s lawyer said state security agents had interviewed his client on suspicion of endangering state security. “He is free to move around, but can’t leave China,” Chen Jinxue said. He said Feng was staying in the Grand International Hotel in Guangzhou with his wife.

Calls to Feng’s room via the hotel operator were not answered Thursday.

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