WASHINGTON (AFP) - US Secretary of State Mike Pompeo on Tuesday (July 14) urged free elections in Hong Kong after China said pro-democracy parties' primaries may have breached a tough new security law.

"Congratulations to Hong Kong's pan-democrats for a successful primary. The Legislative Council election in September should be equally free and fair," Pompeo wrote on Twitter.

In an accompanying statement, Pompeo said the participation of more than 600,000 Hong Kongers in the primary showed "their desire to make their voices heard in the face of the Chinese Communist Party's efforts to suffocate the territory's freedoms."

The Liaison Office, which represents China's government in the semi-autonomous city, called the primary "a serious provocation against the current election system."

Pompeo in the statement voiced "grave concern" about the threat, saying it showed "the Chinese Communist Party's fear of democracy and its own people's free thinking."

China has imposed a law that criminalises subversion and other perceived offences, sending a chill through the city that was promised a separate system before Britain handed back control in 1997.