Satellite photos show what appear to be Chinese armoured personnel carriers and other vehicles parked in a sports complex in the city of Shenzhen, bordering Hong Kong, in what some have interpreted as a threat from Beijing to use increased force against protesters.

Key points: The pictures appear to show more than 100 vehicles in the soccer stadium

The pictures appear to show more than 100 vehicles in the soccer stadium Chinese state media said the Shenzhen exercises had been planned beforehand

Chinese state media said the Shenzhen exercises had been planned beforehand The central Government in Beijing said the protests were beginning to show the "sprouts of terrorism"

The pictures appear to show more than 100 vehicles sitting in and around the soccer stadium at the Shenzhen Bay Sports Centre, just north of the Asian financial hub that has been rocked by more than two months of near-daily street demonstrations.

The images were released by Maxar Technologies, a space technology company known to release satellite images of different parts of the world.

CNN reporters in Shenzhen also confirmed seeing dozens of troop carriers and at least one armoured personnel carrier.

Earlier this week, China's state media reported that forces had been assembled for training exercises in the border city, adding to fears that Beijing might directly intervene in Hong Kong.

Chinese state tabloid Global Times posted a compilation of footage to social media showing military trucks assembling. ( Twitter: Global Times )

Chinese state media has said only that the Shenzhen exercises had been planned beforehand and were not directly related to the unrest in Hong Kong, although they came shortly after the central Government in Beijing said the protests were beginning to show the "sprouts of terrorism".

State broadcaster CCTV aired footage on Wednesday of a mass anti-riot police drill held on August 6 in Shenzhen.

The Shenzhen Public Security Bureau drill involved 12,000 police officers, 50 armoured vehicles, 200 assault vehicles, 1,200 motorbikes, five helicopters, eight boats, and two amphibious vehicles, official state news agency Xinhua said.

The drills covered three subjects: anti-terrorist measures, joint sea and air patrols, and riot prevention, Xinhua added.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 38 seconds 38 s Chinese police take part in mass anti-riot drill in Shenzhen on August 6.

Commenting before the release of the satellite images, President Donald Trump tweeted that US intelligence believed that the Chinese Government was moving troops to its border with Hong Kong and that, "Everyone should be calm and safe!"

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Mr Trump also retweeted footage of troop movements in Shenzhen posted by Alexandre Krauss, a senior political adviser at Renew Europe, which appeared to show military vehicles entering the sports stadium.

"Something extraordinarily bad is about [to] happen," Mr Krauss tweeted.

Beijing has not sent in police or army units from the mainland or mobilised the People's Liberation Army garrison in Hong Kong to quell the unrest.

It is seen as mindful of the devastating effect that would have both on the territory's reputation as a safe and stable place to invest, and as an indication of the Communist Party's failure to win over the hearts and minds of the city's 7.3 million residents.

The Shenzhen Bay Sports Centre prior to the release of the satellite images. ( Google Earth )

Trump suggests meeting with Xi Jinping

Ten weeks of increasingly violent confrontations between police and protesters have plunged Hong Kong into its worst crisis since it reverted from British to Chinese rule in 1997.

President Donald Trump tied a US trade deal with China to the humane resolution of the situation, hours after the State Department said it was "deeply concerned" about reports of movement of Chinese paramilitary forces along the border.

The State Department warned that continued erosion of the territory's autonomy put at risk the preferential status it enjoys under US law.

Mr Trump, in his remarks on Twitter, appeared to suggest a personal meeting with Chinese President Xi Jinping to help resolve the crisis.

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"Of course China wants to make a deal. Let them work humanely with Hong Kong first!" Mr Trump said on Twitter.

"I have ZERO doubt that if President Xi wants to quickly and humanely solve the Hong Kong problem, he can do it. Personal meeting?"

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In his tweets on Wednesday, Mr Trump also said that his delay in 10 per cent tariffs on more than $150 billion in Chinese imports from September to December "will be reciprocated" by China and the "much good will come from the short deferral to December".

Flights resumed on Wednesday amid heightened security at Hong Kong airport, which is one of the world's busiest, after two days of disruptions sparked by protesters swarming the airport.

China's Hong Kong and Macau Affairs Office in Beijing called the behaviour at the airport no different to terrorism and said it must be severely punished.

China used its strongest language yet after Tuesday's incidents, when the protesters seized a reporter from China's Global Times newspaper, a nationalistic tabloid run by the ruling Communist Party's official People's Daily, and harassed the man they believed to be a mainland agent.

In addition to Beijing's condemnation, the People's Daily called for "using the sword of the law" to restore order, and mainland social media users lauded the detained reporter as a hero.

China's state media said forces had assembled for training exercises in the border city ( Maxar Technologies via AP )

AP/Reuters