China suspended US warship visits and sanctioned American NGOs in retaliation for the passage of a bill backing pro-democracy protesters in Hong Kong.

The financial hub has been rocked by nearly six months of increasingly violent unrest demanding greater autonomy, which Beijing has frequently blamed on foreign influence.

Last week, US President Donald Trump signed the Hong Kong Human Rights and Democracy Act, which requires the president to annually review the city's favourable trade status and threatens to revoke it if the semi-autonomous territory's freedoms are quashed.

The move came as the world's two biggest economies have been striving to finalise a "phase one" deal in their protracted trade war.

"In response to the unreasonable behaviour of the US side, the Chinese government has decided to suspend reviewing the applications for US warships to go to Hong Kong for (rest and) recuperation as of today," foreign ministry spokeswoman Hua Chunying said at a regular press briefing.

China had already denied requests for two US Navy ships to dock in Hong Kong in August, without specifying a reason why.