Chinese authorities have slammed the United States for imposing "barbarous" sanctions in response to China's crackdown in Hong Kong, capping a dramatic week of deteriorating relations between the world's two biggest economies.

In the toughest action on Hong Kong since China imposed a sweeping new security law on the territory, yesterday the US sanctioned a group of Chinese and Hong Kong officials - including the city's leader Carrie Lam.

The move came after US President Donald Trump's administration forced Chinese internet giants TikTok and WeChat to end all operations in the US, in a twin diplomatic-commercial offensive set to grow ahead of the US presidential election in November.

China has criticised the sanctions as "barbarous and rude".

"The ill intentions of US politicians to support people who are anti-China and messing up Hong Kong have been clearly revealed," China's Liaison Office in Hong Kong said in a statement.

The Treasury Department announced it was freezing the US assets of Chief Executive Carrie Lam and ten other senior officials, including Luo Huining - the head of the Liaison Office.

It accused the sanctioned individuals of being "directly responsible for implementing Beijing's policies of suppression of freedom and democratic processes".

The move criminalises any US financial transactions with the sanctioned officials.

In a short statement, Mr Luo said he welcomed the blacklisting.

"I have done what I should do for the country and for Hong Kong," he said. "I don't have a dime's worth in foreign assets."

The Hong Kong government described the sanctions as "shameless and despicable".

A city government spokesman said in a statement that Washington was using "Hong Kong as a pawn in its ploy to create troubles in China-US relationship".

China's security law was imposed in late June, following last year's huge pro-democracy protests in Hong Kong, sending a political chill through the semi-autonomous city.

Since then, authorities have postponed elections, citing the coronavirus pandemic, issued arrest warrants for six exiled pro-democracy activists and launched a crackdown on other activists.

US Secretary of State Mike Pompeo said the security law violated promises made by China ahead of Hong Kong's 1997 handover that the city could keep key freedoms and autonomy for 50 years.

"Today's actions send a clear message that the Hong Kong authorities' actions are unacceptable," Mr Pompeo said in a statement.

The US measures come three months ahead of the November election in which Mr Trump, who is behind his rival Joe Biden, is campaigning hard on an increasingly strident anti-China message.

As public disapproval has grown for his handling of the pandemic, Mr Trump has pivoted from his previous focus on striking a trade deal with China to blaming the country for the coronavirus crisis.