China suspended US warship visits and sanctioned American NGOs on Monday 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 favorable trade status and threatens to revoke it if the semi-autonomous territory's freedoms are quashed.

China has suspended US warship visits in retaliation for the passage of a bill backing pro-democracy protesters in Hong Kong. File picture of US Navy personnel during a visit to Hong Kong

Last week US President Donald Trump signed the Hong Kong Human Rights and Democracy Act

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

'In response to the unreasonable behavior 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.

'Operationally, from a military point of view, it doesn't really make a difference for the US, as they can use many naval bases in the region,' Michael Raska, a security researcher at Singapore's Nanyang Technological University, told AFP.

However, it 'sends a signal that US-China tensions will continue to deepen,' Raska said.

The last US navy ship to visit Hong Kong was the USS Blue Ridge in April.

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

J. Michael Cole, a Taipei-based senior fellow with the Global Taiwan Institute, said the move was 'mostly symbolic' but yet another sign of the 'tit-for-tat escalation which is poisoning the bilateral relationship.'

Hua said they would also apply sanctions to a number of US-based NGOs, although failed to give any specifics over the form the measures would take.

Sanctions will apply to NGOs that had acted 'badly' over the recent unrest in Hong Kong, she said, including the National Endowment for Democracy, Human Rights Watch and Freedom House.

Yesterday, pro-democracy protesters in Hong Kong wore Trump masks and waved US flags during a March of Gratitude rally to say thank you for their support

Police fire tear gas during a protest at Hung Hom area in the latest round of protests which have rocked Hong Kong for six months

There was 'already a large amount of facts and evidence that make it clear that these non-governmental organisations support anti-China' forces and 'incite separatist activities for Hong Kong independence', Hua said.

She accused them of having 'great responsibility for the chaotic situation in Hong Kong'.

Protesters in Hong Kong are pushing for greater democratic freedoms and police accountability, but the city's pro-Beijing leadership has refused any major political concessions.

The increasingly violent rallies have hammered the retail and tourism sectors, with mainland Chinese visitors abandoning the city in droves.

The city's finance chief warned Monday that Hong Kong is set to record its first budget deficit in 15 years.