More than 140 academics and former diplomats have petitioned Chinese President Xi Jinping to release detained Canadians Michael Kovrig and Michael Spavor.

The request was outlined in an open letter released on Tuesday with signatories from 19 countries. They included Christopher Patten, the last British governor of Hong Kong and current chancellor of Oxford University; Volker Stanzel, the former German ambassador to China; and five former Canadian ambassadors to China.

The letter praised the work of both Kovrig and Spavor to promote cultural and intellectual exchange between the West, China and North Korea.

The letter said the pair's detention "send a message that this kind of constructive work is unwelcome and even risky in China."

Kovrig, a former Canadian diplomat, was working for non-profit International Crisis Group when he was detained on December 10 in China.

Spavor is the director of the Paektu Cultural Exchange, which promotes North Korean tourism and investment. He was detained on December 13.

Both men are held on charges of "endangering national security" although they are widely believed to be held in retribution for the arrest of Huawei chief financial officer Meng Wanzhou on December 1 in Canada.

Meng was detained while changing flights in Vancouver. She is wanted by the United States for allegedly violating US sanctions on Iran.

Her arrest is also believed to be linked to the death sentence handed down to Canadian Robert Lloyd Schellenberg for drug-smuggling in China in mid-January.