The United States raised its travel warning for Hong Kong, urging increased caution by visitors to the Chinese territory in the face of what it described as civil unrest after months of sometimes violent street protests.

The protests in the Asian financial hub began with opposition to a now-suspended extradition law and have evolved into a direct challenge to the city's government and calls for full democracy.

"The protests and confrontations have spilled over into neighborhoods other than those where the police have permitted marches or rallies," said the advisory, posted on the website of the U.S. state department on Wednesday.

"These demonstrations, which can take place with little or no notice, are likely to continue," it added. The advisory was raised to level two on a four-point scale.

Australia also warned its travelers in an updated advisory.

The protests pose a grave popular challenge to Chinese President Xi Jinping since he came to power in 2012. Xi is also grappling with a debilitating trade war with the United States and a slowing economy.

China's Foreign Ministry lodged stern representations with the United States, urging U.S. officials to stop sending wrong signals to the "violent separatists" in Hong Kong.

The protests have left Hong Kong facing its worst crisis since it returned to China from British rule in 1997, the head of China's Hong Kong and Macau Affairs office said.

More protests are planned in several districts across the city this weekend, starting on Friday, with demonstrators also planning a three-day rally at the city's international airport.

Police warned activists to protest peacefully and said they had detained three more people, raising the number arrested to nearly 600 since protests began in June, the youngest aged 13.

Protesters want the city's embattled leader Carrie Lam to categorically withdraw the extradition bill, and an independent inquiry into the government and the police handling of the controversy.

Lam, who says the bill is dead but has not withdrawn it, visited some districts on Wednesday to speak with residents and inspect a police station recently targeted by protesters.

The government would put forward measures to improve people's livelihoods, she said in a statement after the visit.