US Secretary of State Mike Pompeo on Monday demanded that China free two Canadians who were charged with spying following a row involving all three nations.

"The United States stands with Canada in calling on Beijing for the immediate release of the two men and rejects the use of these unjustified detentions to coerce Canada," Pompeo said in a statement.

Pompeo said the United States was "extremely concerned" by the charges and also urged China to allow consular access, saying there has been no contact with the pair for nearly six months.

Former diplomat Michael Kovrig and businessman Michael Spavor were detained in December 2018, nine days after Huawei executive Meng Wanzhou was arrested in Canada on a US warrant.

Canadian Prime Minister Justin Trudeau, who called Friday for the two men's release after the charges were unveiled, has said that China explicitly linked their arrests to the Meng case.

Relations between Canada and China have hit rock bottom over the arrests. Beijing has blocked billions of dollars in Canadian agricultural exports.

Meng is the daughter of the founder of Huawei, the telecom giant which the United States, with limited success, is urging all nations to shun on security grounds.

She is living in a mansion in Vancouver while she is out on bail.