File photo. Photograph:( AFP )
US Congress also passed legislation banning sales of tear gas, rubber bullets and other equipment to Hong Kong.
US President Donald Trump on Wednesday signed the Hong Kong Human Rights and Democracy Act after it was passed unanimously by both houses of Congress.
In a statement, Trump said he "hoped the leaders and representatives of China and Hong Kong will be able to amicably settle their differences," while reiterating his "respect" for Chinese President Xi Jinping.
As the US president signed the bill, China threatened "firm countermeasures" over the Hong Kong bill while expressing "extreme regret".
The US Congress also passed legislation banning sales of tear gas, rubber bullets and other equipment to Hong Kong which was widely used by authorities against protesters in the city.
Hong Kong has witnessed months of anti-government protests with citizens of the city primarily students clashing regularly with police. In recent weeks, Hong Kong's Polytechnic University has become the epicentre of violent protests between authorities and students with the siege being lifted recently.
The passing of the Democracy act comes amid the US-China trade war with Trump eager to make a deal with the Xi government.
Meanwhile, China's foreign ministry said Trump's backing of protesters in Hong Kong was a serious interference in Chinese affairs and US efforts were "doomed to fail".
"The nature of this is extremely abominable, and harbours absolutely sinister intentions," the foreign ministry said in a statement.
China's foreign minister, Wang Yi had said earlier that the bill was as "naked interference in China's internal affairs" as China summoned the US ambassador to Beijing to raise strong objections over the bill.
China's vice foreign minister told the US ambassador that the United States would bear "bear all the consequences" if the bill went through.