China has condemned violent clashes in the Hong Kong protests as "behaviour that is close to terrorism" - in a sign of its rhetoric hardening.
After days of peaceful protests at Hong Kong International Airport, clashes with police broke out on Tuesday night.
Video showed an officer drawing his gun on protesters who beat him with his own truncheon during the disturbance.
It is the second time in a week that Chinese officials have publicly likened the protests to terrorist activity.
Some observers believe that the repeated use of such language suggests that China is losing patience with the protesters, and could increase the likelihood of an intervention from Beijing.
The US said it was "deeply concerned by reports of Chinese paramilitary movement along the Hong Kong border" and urged China to respect Hong Kong's high degree of autonomy.
However, most analysts consider that at this stage, a direct military intervention is still unlikely.
The former British colony has a special status, with its own legal system and judiciary, and rights and freedoms not seen in mainland China. However, many activists believe this is now under threat.
Millions of Hong Kong citizens have taken part in 10 weeks of anti-government protests, demanding democratic reform and an investigation into alleged police brutality. While many of the demonstrations were peaceful, an increasing number have ended in violent clashes with police.
The latest protest, an "occupation" of the airport, led to hundreds of flights being cancelled after protesters escalated their action, though normal service has mostly resumed.
But China seized on Tuesday's brief outbreak of violence as evidence of "violent crimes" that "breached legal and moral bottom lines".
What happened at the airport on Tuesday?
The airport had been the site of mostly peaceful protests since last Friday - but on Tuesday, protesters blocked travellers from accessing flights, using luggage trolleys to build barriers, and staging a mass sit-down.
Some protesters held signs apologising to passengers for the inconvenience caused by their demonstrations.
Two incidents, however, sparked clashes with police.
At least two men were set upon by protesters, accused of being undercover police officers - a fear prompted after the police admitted they had deployed officers disguised as anti-government protesters.
One man, who was tied up with zip ties, was later revealed to be Fu Guohao, a reporter for Chinese state media outlet the Global Times - though it is not clear if he identified himself.
Appearing on state television in China the next day, Mr Fu said he "didn't behave illegally or controversially. I don't think I should be treated violently".
Police, wearing riot gear and brandishing truncheons, arrived at the airport and clashed with protesters.
The second major incident caught on camera involved an officer who reportedly manhandled a woman among the protesters. But instead, his own truncheon was taken from him and he was beaten with it after being rushed into a corner.
He frantically drew his gun and pointed it at the crowd to disperse them, before being rescued by his fellow officers.
Tensions between protesters and police have ramped up further in recent days, after police were seen firing pepper ball rounds on protesters at close range, and firing tear gas in an enclosed train station, during protests on Sunday.
What have authorities said about Tuesday's clashes?
Hong Kong police said the officer's life had been "under great danger" and insisted he had only drawn his gun "out of emergency and necessity" and "exercised great restraint".
Meanwhile, Chinese media are actively promoting the video of the reporter's ordeal in mainland China, where news of the Hong Kong demonstrations has been carefully managed, says the BBC's Asia-Pacific editor Michael Bristow.
A statement released by the Hong Kong affairs office of China's state council condemned the violence in fierce terms, describing the demonstrators as "radical violent elements" who had attacked two people from mainland China and "aimed lasers at their eyes".
The statement alleged they had "encircled a police officer and snatched his baton", without providing any additional context.
Police likened the treatment of the men whom protesters had captured to "torture" and said they had arrested five people. The Hong Kong government called the "violent acts... outrageous" and said that they had "overstepped the bottom line of a civilised society".
What about international reaction?
Officials in the US viewed events differently.
Members of the House Foreign Affairs committee issued a joint statement expressing concern that China might consider "brutally putting down peaceful protests" and lauding "the brave efforts of Hong Kong people" in their demonstrations.
The US state department also issued a travel advisory for Hong Kong on Wednesday, alongside its expression of concern over "paramilitary movement" on the border. It urged all sides to refrain from violence.
Separately, an image released by satellite imaging firm Maxar Technologies showed what appeared to be military or security vehicles gathering out of public view inside a sports stadium in Shenzhen, which borders Hong Kong. The photo was taken on 12 August, the firm said.
Overnight, protest groups issued online apologies for the violence at the airport, saying they were "scared" and appealed for help.
"We're deeply sorry about what happened yesterday," a banner held up in the arrivals hall on Wednesday morning said, according to Reuters news agency.
"We were desperate and we made imperfect decisions. Please accept our apologies."
Some also handed out apology leaflets and chocolate to people arriving at the airport's train station.