The family of Huseyin Celil, a Canadian citizen imprisoned in China, says they have no idea where he's being held.

In a letter to Prime Minister Stephen Harper, sent jointly with Amnesty International, Celil's family is urging the government to do more to ensure his safety.

"Prime minister, we call on you to renew and fortify Canada's efforts to ensure Mr. Celil's human rights are protected," the letter states.

"We appreciated the concern and action demonstrated by the government in the past, including by you personally, but are concerned Canada's attention to Mr. Celil's fate appears to have waned."

CTV's Steve Chao, reporting Friday from Beijing, said the big concern right now surrounds Celil's whereabouts.

"As of last fall, his sister was still being allowed to visit him in a prison but then earlier this month, when she went to the same prison, government authorities told her that he had been moved but they refused to let her know where exactly he had been moved to," said Chao. 

Celil, who belongs to the Uighur Muslim minority of far western China, holds Canadian citizenship. The Chinese-born man came to Canada via Uzbekistan and Turkey after escaping from a Chinese jail in 2000.

In late March 2006, Celil visited Uzbekistan and was arrested and returned to China.

Chinese authorities claim that militants among the Uighurs -- Turkic-speaking Muslims -- are backing a violent Islamic separatist movement in an attempt to set up an independent state of "East Turkistan.''

China's line has long been that because Celil was born in China they will not recognize his Canadian citizenship.

"This is against international agreements however the Chinese government says this is their matter... and they do not want Canada involved," said Chao.

In April 2007, a Chinese court found Celil guilty for the two crimes of "separating China and ... organizing, leading and participating in terrorist groups, organizations."

He was given a life sentence for the crimes. In July 2007, a Chinese court rejected Celil's appeal.

Celil's family says he is being held in jail because he is a human rights activist.

Canadian diplomats have never been granted access to Celil, said Chao.