BEIJING (Reuters) - China said on Thursday that it blocked an Australia-based Chinese academic from taking a flight to Sydney this week to “safeguard national security” as fellow professors called for Beijing to allow him to return home.

Feng Chongyi, a long-service associate professor of China studies at the University of Technology Sydney, was blocked from going home after a trip to China on Sunday, according to the Australian government.

Feng was held for questioning last week in Kunming, the provincial capital of southwestern Yunnan, then barred from boarding a flight to Sydney from Guangzhou on Friday, sources familiar with the situation told Reuters.

The news of his being blocked from leaving came as Chinese Premier Li Keqiang finished up a five-day official visit to Canberra and Sydney espousing free trade and closer economic ties between Australia and China.

“In order to safeguard China’s national security, the relevant departments took measures in accordance with law against Chinese citizen Feng Chongyi to prevent him from leaving the country,” Chinese Foreign Ministry spokesman Lu Kang said at a regular press briefing in Beijing.

“Feng Chongyi is a Chinese citizen... If this was an Australian citizen, and Australian authorities wanted to open an investigation, then (the citizen) would also be obliged to comply.”

Feng, an Australian permanent resident who retains his Chinese passport, entered China three weeks ago and had been traveling with his wife, an Australian national.

Australia’s Department of Foreign Affairs and Trade said on Thursday that the government was “monitoring developments closely and has raised this case with senior Chinese officials”.

When reached by Reuters on Thursday, Feng directed questions to his lawyer, Chen Jinxue, due to the sensitivity of his situation.

Chen told Reuters that police had previously suggested Feng was being prevented from leaving China on national security grounds but had not elaborated.

He said Feng remained at his hotel in Guangzhou and state security officers were interrogating him daily for two to three hours at a time. He said state security had instructed Feng not to reveal details over what he was being questioned about.

“The definition of national security is extremely broad,” Chen said. “What exactly did he do? If he can’t leave the country, they should at least give him something in writing.”

Feng had been meeting with fellow academics and intellectuals while in China, as well as human rights lawyers as part of his academic research.

An open letter addressed to Chinese President Xi Jinping and Prime Minister Li signed by dozens of internationally respected China Studies scholars from Australia, the United States, Britain and Hong Kong was also circulated on Thursday “respectfully requesting” Feng’s release.

The letter said they were “disturbed” that a fellow researcher was “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,” it said.