Hong Kong police today entered a ransacked campus of the Hong Kong Polytechnic University. — AFP/VNA Photo

BEIJING — China warned today that it was ready to take "firm countermeasures" against the United States after President Donald Trump signed a law supporting protesters in Hong Kong.

"The nature of this is extremely abominable, and harbours absolutely sinister intentions," the foreign ministry said in a statement, without specifying what measures Beijing might take.

On Wednesday Trump signed the law, likely angering Beijing just as Washington was hoping to ease the long-running US-China trade war.

Trump had seemed reluctant to sign the bill but with almost unanimous US congressional support for the measure, he had little political room to maneuver.

In a statement, Trump spoke of "respect" for Chinese President Xi Jinping and said he hoped the "leaders and representatives of China and Hong Kong will be able to amicably settle their differences."

A week ago, China's foreign minister, Wang Yi, condemned the bill as "naked interference in China's internal affairs."

Just on Tuesday, the foreign ministry said it had summoned the US ambassador to Beijing to warn that the United States would "bear all the consequences" if the bill went through.

The Hong Kong Human Rights and Democracy Act, which won rare and overwhelming bipartisan support in both houses of Congress, supports Hong Kong's protesters.

It requires the US president to annually review the city's favorable trade status and threatens to revoke it if the semi-autonomous territory's freedoms are quashed.

Congress also passed legislation banning sales of tear gas, rubber bullets and other equipment used by Hong Kong security forces in putting down the protests, which are now in their sixth month.

Meanwhile, Hong Kong police today entered a ransacked university campus where authorities faced off for days with barricaded protesters, looking for petrol bombs and other dangerous materials left over from the occupation.

The Hong Kong Polytechnic University became the epicentre of the territory's increasingly violent protest movement when clashes broke out on November 17 between police and protesters armed with bows and arrows as well as Molotov cocktails.

The standoff settled into a tense stalemate during which hundreds fled the campus – some making daring escapes, others caught and beaten by officers during failed breakouts – leaving a dwindling core of holdouts surrounded by police cordons.

But in recent days, the last few people barricaded in the campus seemed to disappear.

University staff said they were only able to find a single protester on campus and reporters there struggled to see any major presence in the last 48 hours. — AFP