Hong Kong: Beijing on Wednesday slammed "terrorist-like" attacks on its citizens by pro-democracy protesters in Hong Kong during a second day of chaos at the city's airport, as the United States expressed concern over apparent Chinese troop movements at the border.

The rallies, which had paralysed one of the world's busiest travel hubs, ended with ugly clashes on Tuesday night that included protesters beating two men.

The Chinese government immediately seized on the attacks to intensify its drumbeat of anger and intimidation against the protesters, who have staged 10 weeks of relentless rallies to demand greater freedoms in the semi-autonomous city.

"We express the strongest condemnation of these terrorist-like actions," said Xu Luying, spokeswoman at the Hong Kong and Macao Affairs office of the State Council, who called the two men who were beaten "mainland China compatriots."

It was the second time this week that China had sought to frame the protests as "terrorism" -- part of a pattern of increasingly ominous warnings that have raised fears it may deploy force to quell the unrest.

China on Monday said there were signs of "terrorism emerging", as state media published a video of armoured personnel carriers driving towards the border with Hong Kong.

US President Donald Trump added to fears Beijing may stage a military intervention to end the unrest, saying on Tuesday his intelligence had confirmed Chinese troop movements toward the Hong Kong border.

"I hope it works out for everybody including China. I hope it works out peacefully, nobody gets hurt, nobody gets killed," Trump said.

On Wednesday, the US State Department said it was "deeply concerned" by the apparent troop movements, and called on Beijing to "adhere to its commitments" to "allow Hong Kong to exercise a high degree of autonomy." The people power movement, which has seen millions take to Hong Kong's streets, was sparked by opposition to a planned law that would have allowed extraditions to mainland China.

It quickly evolved into a much broader campaign for democratic freedoms.

Under a 1997 deal that saw Hong Kong return to China from British colonial rule, the city is meant to have far greater liberties than those allowed on the mainland.

The protesters have enjoyed widespread support throughout most sectors of Hong Kong's society.

But the beating of the two men at the airport on Tuesday, as well as the blocking of passengers from boarding flights, could spark a backlash.

The movement has no public leaders, with protests organised anonymously over social media and chat apps.

One group sought Tuesday to address the concerns that the airport rallies had gone too far.

"After months of prolonged resistance, we are frightened, angry and exhausted. Some of us have become easily agitated and over-reacted last night," the group said in a statement.

"For this we feel pained and dispirited and would like to express our most sincere apologies." Demonstrators turned on the two men, fuelled by suspicions within their ranks about undercover police or spies.

The first man was held for about two hours and assaulted before being led away in an ambulance.

Riot police briefly deployed pepper spray and batons to subdue protesters while they escorted the vehicle away.

Another man, wearing a yellow journalist vest, was surrounded, zip-tied and then beaten by a small group who accused him of being a spy.

Hu Xijun, the editor of China's state-controlled Global Times tabloid -- which has vociferously condemned the protests -- tweeted that the man was a journalist for the paper.

In another ugly scene, a group of protesters ganged up on a policeman and beat him. They stopped their attack when the policeman pulled his gun and pointed it at them, but did not fire.

On Wednesday, only a handful of protesters rallied at the airport and many flights were operating as scheduled.

Around 100 demonstrators later gathered in the city's working class Sham Shui Po neighbourhood, shining lasers at a police station. Officers used tear gas to disperse them.

Hong Kong's flag carrier Cathay Pacific -- which has come under huge pressure from Beijing to clamp down on staff supportive of anti-government rallies -- said Wednesday it had sacked two pilots.

"One is currently involved in legal proceedings. The other misused company information" about a flight, it said.

The airline said last week it had suspended a pilot accused of rioting after allegedly participating in protests.

It also said it had fired two ground staff, without specifying why. Local media reported that they were accused of leaking travel details of a Hong Kong police football team.

The city's leader, Carrie Lam, warned Tuesday of dangerous consequences if escalating violence continued and said the hub was being "pushed into an abyss."