Canada has asked for an explanation from senior Chinese officials after the Chinese embassy in Ottawa retracted the visas of two journalists who planned to accompany Paul Martin, the Prime Minister, on his trip to Asia.

The two visas had been approved and a representative from the television network was to pick them up at the Prime Minister's Office yesterday when the news came the visas had been retracted, said Joe Wang, the president of the Canadian arm of New Tang Dynasty TV.

The three-year-old Chinese language satellite network has four stations in Canada, including one in Ottawa and about 50 worldwide.

Mr. Wang said he was shocked by the retraction. The Prime Minister's Office wants answers as well.

The question was put to the Chinese ambassador yesterday, but no satisfactory answer was given, said Melanie Gruer. She has been assistant director of communications in the Prime Minister's Office for a year, and has never heard of a visa being retracted.

Ms. Gruer said the Prime Minister's Office has called a senior official from the Chinese embassy to ask that the visas be reinstated. Canada's ambassador to China is to make the same request today in Beijing.

About 35 to 40 journalists were to accompany Mr. Martin on the overseas trip, which will include stops in Thailand, Sri Lanka, India, Japan, China and Hong Kong. The visit to China, which begins Jan. 20, is scheduled to last two days.

Mr. Wang said the journalists, reporter Daniel Zhu and camera operator David Ren, are both Canadian citizens who were born in China.

The Chinese government is opposed to the New Tang Dynasty network because it has reported on issues such as SARS and the persecution of practitioners of Falun Gong, Wang said. Both of the journalists have worked on stories critical of the Chinese government's policies.

The not-for-profit network, which is available on satellite, produces general interest programming in Chinese. The visa issue is a clear example of China's interference with the media, Mr. Wang said .

"Our missions is to report truthfully to the Chinese community," he said. "We were pumped up about the Chinese trip. Then this happened."

An inquiry with the press office of the Chinese embassy in Ottawa was not answered yesterday.

In another development, advocates are calling on Mr. Martin to champion human rights protection in China with the same vigour as Canada has promoted closer trade ties with the Communist nation.

Amnesty International and groups representing Chinese dissidents and Tibetans demanded in an open letter to the Prime Minister that he abandon Canada's long-standing policy of delinking human rights from trade issues.

The view held by successive governments that human rights will inevitably improve as China's economic ties with the West grow has not borne fruit in delivering democratic freedoms, the coalition said.

"I suppose it has been successful in terms of increased trade, but it has not delivered the goods in terms of improving the human rights situation," said Alex Neve, secretary general of Amnesty International.

"Torture continues, executions go on, ethnic and religious minorities still face imprisonment ... You can't leave human rights to the whim and chance of market forces."