The whereabouts of a Chinese professor known for his views critical of the government remained unknown Friday, two days after police interrupted his radio interview with US government broadcaster Voice of America.

Sun Wenguang was speaking to the station on Wednesday night when he says that half a dozen officers barged into his apartment in the eastern city of Jinan. He can be heard exclaiming that "I have my freedom of speech" just before the line goes dead.

VOA said that Sun had not responded to attempts to contact him, but that sources it did not identify said he was being held in a military-run hotel in Jinan.

A former professor of physics, the 84-year-old Sun has long been critical of China's communist leadership, most recently protesting China's vast expenditures on development projects abroad at a time when many Chinese remain poor.

That's seen as a knock on the president and party leader Xi Jinping's signature "Belt and Road" initiative that has earmarked an estimated $1 trillion for ports, power plants and other projects linking China to parts of Asia, Europe, Africa and beyond. The initiative has recently run into headwinds as the countries involved balk at the massive debts they stand to incur.

Before the line goes dead, Sun can be heard saying "throwing money around like this is of no benefit to our country and society."

Sun was also an early co-signer of "Charter 08," a call for democratic reform whose co-author, Liu Xiaobo won the Nobel Peace Prize but died last year while serving a sentence for subversion.

Sun's views have brought official retribution ranging from a seven-year prison sentence to being denied a passport and having his pension cut.

Responding to Sun's apparent detention, U.S. Rep. Chris Smith from New Jersey issued a statement Thursday calling for the professor's immediate release.

"The Chinese and American people must continue to work toward a day when someone like Prof. Sun can openly share his opinions, via a free press, without fear of reprisal," Smith said.