Trump angers China by signing bill supporting pro-democracy Hong Kong protesters

US President Donald Trump has signed a law supporting pro-democracy protesters in Hong Kong, drawing a furious reaction from China which on Thursday warned of "firm countermeasures".

The legislation was approved unanimously by the US Senate and by all but one lawmaker in the House of Representatives last week.

Trump signed the Hong Kong Human Rights and Democracy Act of 2019 into law on Wednesday.

The law would require the State Department to certify once a year that Hong Kong is sufficiently autonomous to retain its special US trading consideration a status that helps its economy.

It also threatens sanctions on Chinese and Hong Kong officials deemed responsible for human rights abuses in Hong Kong, escalating tensions between China and the US.

Hong Kong is a semi-autonomous territory which operates under the 'one country, two systems' principle a structure that grants the city's citizens some degree of financial and legal independence from the mainland.

The city has been shaken by massive, sometimes violent, protests initially organised to oppose a now-suspended bill that would have allowed extraditions to mainland China. These protests have now turned into a wider movement for democratic reforms.

Trump in a statement said, "I signed these bills out of respect for President Xi (Jinping), China, and the people of Hong Kong."