The United States on Monday imposed sanctions on four more officials accused of curbing freedoms in Hong Kong, vowing accountability over China's clampdown in the city.

Edwina Lau, head of the National Security Division of the Hong Kong Police Force, was among the officials who will be barred from traveling to the United States and whose US-based assets, if any, will be frozen.

"These actions underscore US resolve to hold accountable key figures that are actively eviscerating the freedoms of the people of Hong Kong and undermining Hong Kong's autonomy," Secretary of State Mike Pompeo said in a statement.

Other officials hit by sanctions Monday include Li Jiangzhou, deputy director of a Hong Kong office for "safeguarding national security."

The United States has already imposed similar sanctions on Hong Kong's top leader, Carrie Lam, who is an ally of Beijing.

Lam has tried to downplay the impact but acknowledged that she had trouble with a credit card after the sanctions.

The US pressure comes after China forged ahead with a tough security law that bars subversion in the financial hub, which has witnessed major pro-democracy protests.

China's clampdown comes despite its promises that it would ensure a separate system for Hong Kong before Britain handed over its then colony in 1997.

Beijing has kept chipping away at dissent in Hong Kong, where pro-democracy lawmakers threatened Monday to resign en masse if four of their colleagues are disqualified from seeking office.