Washington has warned US citizens to "exercise increased caution" when travelling to Hong Kong, as protesters announced three days of new demonstrations at the city's airport.
The US State Department raised the travel advisory level due to what it termed as "civil unrest", after months of street protests that turned violent at times.
"The protests and confrontations have spilled over into neighbourhoods other than those where the police have permitted marches or rallies," said the advisory, which was first posted on the US Consulate General Hong Kong and Macau's website on Wednesday (Aug 7).
The advisory was raised to level two on a four-point scale which meant travellers were urged to "exercise increased caution".
The protests in the Asian financial hub began with opposition to a now-suspended extradition Bill and have evolved into a direct challenge to the government and calls for full democracy.
"Since June 2019, several large-scale and smaller political demonstrations have taken place in various areas of Hong Kong," said the advisory.
"Most have been peaceful, but some have turned confrontational or resulted in violent clashes.
"The protests and confrontations have spilled over into neighbourhoods other than those where the police have permitted marches or rallies.
"These demonstrations, which can take place with little or no notice, are likely to continue," it said.
The US warning comes after countries including Australia, Britain, Ireland, Singapore and Japan issued heightened travel warnings for Hong Kong.
On Tuesday China gave its sternest warning yet to the pro-democracy demonstrators, but the protests are expected to continue, with demonstrators planning three days of rallies at the city's airport from Friday afternoon.
Protesters circulated brightly-coloured pamphlets online ahead of the airport demonstrations, aiming to help tourists understand events, in which they vowed never to surrender.
"Dear travellers, please forgive us for the 'unexpected Hong Kong'. You're arrived in a broken, torn-apart city, not the one you have once pictured. Yet the city you imagined is exactly what we are fighting for," the pamphlets said.
On Monday, a general strike called by protesters saw more than 160 flights cancelled and transport in the city paralysed.
Responding to CNA's queries, Hong Kong's flagship carrier Cathay Pacific said it was aware of plans for a public assembly at the airport between Friday and Sunday.
"Our flights to and from Hong Kong International Airport are currently operating as normal," it added, but warned passengers to allow sufficient time to travel to the airport, as there may be traffic congestion in the city.
"Additionally, there may be added entrance procedures at the airport – to save time, we highly recommend checking in online."
The Airport Authority Hong Kong (AA) told CNA it was aware of calls for a public assembly at the airport on Friday.
"The AA understands that there is no application so far for these activities," it added.
"The AA will work closely with its business partners, and has contingency measures to ensure smooth airport operations."
It urged passengers to "allow sufficient time for travelling to the airport" and to check flight status information before coming to the 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.
Three masked activists, who did not give their names, held a news conference on Thursday, their second this week and broadcast on domestic television channels, to criticise what they called arbitrary arrests and police use of tear gas.
"The continuation of such attempts at spreading fear and suppressing the freedom of press will eventually backfire on the government itself," one activist told the Citizens' Press Conference, a platform protesters are using to voice concerns over the situation in Hong Kong.
"The ultimate victim of these tactics will be the police force's crumbling public image," said the activist, who spoke in English.
On July 26, protesters gathered at the airport for a demonstration, sharing their message with tourists arriving at the busy transport hub. The protest was peaceful and did not interrupt flights.
But Cathay Pacific has seen bookings drop as the crisis continues in Hong Kong, chairman John Slosar warned Wednesday.
"The protests in Hong Kong reduced inbound passenger traffic in July and are adversely impacting forward bookings," he said.
Hong Kong is facing its worst crisis since it returned to China from British rule in 1997 because of the protests, the head of China's Hong Kong and Macau Affairs office said on Wednesday.
The protests pose the biggest 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.