Donald Trump has urged China to "humanely" resolve the violent stand-off with pro-democracy protestors in Hong Kong, on the back of growing concerns that Beijing is considering direct intervention in the crisis.

Images taken by AFP on Thursday showed thousands of Chinese military personnel waving red flags and parading at a sports stadium in the city of Shenzhen, just across the border from Hong Kong.

Dozens of armoured personnel carriers and supply trucks were also parked nearby.

Chinese state-run media reported this week that the elements of the People's Armed Police (PAP), which is under the command of the Central Military Commission, were assembling in Shenzhen.

The parade comes as the US president linked a possible trade deal with Beijing to a peaceful resolution to the political unrest that has roiled the semi-autonomous Chinese city for 10 weeks.

Washington has become increasingly alarmed by Chinese security forces gathering near the border with Hong Kong as the protests show no signs of abating and Beijing intensifies its drumbeat of intimidation against a movement pushing for democratic reforms.

"Millions of jobs are being lost in China to other non-Tariffed countries. Thousands of companies are leaving. Of course China wants to make a deal. Let them work humanely with Hong Kong first!" Trump wrote on Twitter, in the first clear indication that the trade deal could be threatened by how Beijing reacts to the protests.

"I have ZERO doubt that if President Xi wants to quickly and humanely solve the Hong Kong problem, he can do it," Trump said in a subsequent tweet, suggesting a "personal meeting" with Chinese counterpart Xi Jinping.

The Hong Kong protests were sparked by opposition to a plan to allow extraditions to the mainland, but have since morphed into a wider — sometimes violent — call for democratic rights.

The movement represents the greatest challenge to Beijing's authority since the city was handed back by the British in 1997 under a deal that allowed it to keep freedoms that many Hong Kongers feel are being eroded.

Weekend rallies after airport violence

Activists are planning another series of mass rallies this weekend in a bid to show their cause still maintains broad public support despite violent scenes during a disruptive occupation of the airport.

On Tuesday, chaos erupted at one of the world's busiest transport hubs as protesters physically stopped travellers from boarding flights, battled riot police and assaulted two men they accused of being Chinese infiltrators.

Beijing — which has refused to grant any concessions to the protest movement — has seized on the airport violence, with state-media churning out a deluge of condemnatory articles, pictures and videos.

"The radicals' violent attacks on innocent citizens are tantamount to an act of terrorism that should be condemned in the strongest terms possible," Xinhua wrote in a commentary on Thursday.

Until the airport protest, hardcore demonstrators had largely focused their anger towards the police, with whom they have fought weekly battles, or state institutions such as the city's parliament and Beijing's main office in the city.

The chaotic scenes inside the airport have prompted some soul-searching within the largely leaderless movement over whether that violence has undermined their cause.

As some groups sent out apologies, messaging forums used to organise protests have filled with calls to support a planned rally on Sunday organised by the Civil Human Rights Front — a group that advocates non-violence and has previously managed to get colossal crowds out onto the streets.

"An urgent call from the peaceful, the rational and the non-violent: the whole world come out on August 18!" read one popular thread on the Reddit-like LIHKG forum used by protesters.

Hands-off approach

Trump's tweets on Hong Kong appeared to signal something of a change in his approach to the city.

In recent days he has come under fire from both sides of the political aisle for shying away from the issue, avoiding criticising Beijing even as he cited US intelligence reports of Chinese forces moving to the territory's border.

China has portrayed the protests as a foreign-funded attempt to destabilise the motherland rather than a popular revolt against its policies.

Washington and Beijing have imposed tariffs on $360 billion in two-way trade, but Trump has delayed tariffs on electronic goods from China, giving investors hope for a detente in the trade conflict.

The introduction of Hong Kong as a potential bargaining chip in those talks could produce a further wrinkle.

Beijing has made it clear it is in no mood to offer concessions to Hong Kong's protesters, whose demands include an independent inquiry into the police's use of tear gas and rubber bullets as well as a long-demanded right to elect the city's leader.