* Trump ties Hong Kong protests to U.S.-China trade deal
* China reiterates that protests are approaching 'terrorism'
* Protests pose major challenge for Chinese leader Xi
* U.S. worried by reports Chinese police massing on border
* Airport reopens but more protests planned across city (Recasts, adds details)
By Farah Master
HONG KONG, Aug 15 (Reuters) - Hong Kong braced on Thursday for more mass demonstrations through the weekend, as China again warned against foreign interference in the city's escalating crisis and as mainland paramilitary forces conducted exercises just across the border.
Western governments, including the United States, have stepped up calls for restraint, following ugly and chaotic scenes at the city's airport this week, which forced the cancellation of nearly 1,000 flights and saw protesters set upon two men they suspected of being government sympathisers.
The airport, one of the world's busiest, was returning to normal but under tight security after thousands of protesters had jammed its halls on Monday and Tuesday nights, part of a protest movement Beijing has likened to terrorism.
Across a bridge linking Hong Kong's rural hinterland with the booming mainland city of Shenzhen, hundreds of members of the paramilitary People's Armed Police conducted exercises at a sports complex in what was widely seen as a warning to protesters in Hong Kong.
The police could be seen carrying out crowd-control exercises, and more than 100 dark-painted paramilitary vehicles filled the stadium's parking lots.
Chinese state media had first reported on the exercises on Monday, prompting U.S. concerns they could be used to break up the protests. However, several western and Asian diplomats in Hong Kong told Reuters Beijing has little appetite for putting the PAP or the People's Liberation Army onto Hong Kong's streets.
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, and police tactics have been toughening.
The protests represent the biggest populist challenge for Chinese President Xi Jinping since he came to power in 2012 and show no immediate signs of abating.
Late on Wednesday night, police and protesters faced off again on the streets of the financial hub, with riot officers quickly firing tear gas.
Seventeen people were arrested on Wednesday, bringing the total detained since June to 748, police told a news conference, adding that police stations have been surrounded and attacked 76 times during the crisis.
U.S. President Donald Trump tied a U.S.-China trade deal to Beijing resolving the unrest "humanely", and suggested he was willing to meet Xi to discuss the crisis.
"I have ZERO doubt that if President Xi (Jinping) wants to quickly and humanely solve the Hong Kong problem, he can do it. Personal meeting?" Trump said on Twitter.
The U.S. State Department said it was deeply concerned over reports that Chinese police forces were gathering near the border with Hong Kong and urged the city's government to respect freedom of speech.
It also issued a travel advisory urging U.S. citizens to exercise caution in Hong Kong. China has frequently warned against what it regards as outside interference in an internal issue.
Other foreign governments urged calm. France called on city officials to renew talks with activists, while Canada said China should handle the protests with tact.
The Civil Human Rights Front, which organised million-strong marches in June, has scheduled another protest for Sunday.
The protesters have five demands, including the complete withdrawal of a now-suspended extradition bill that would have allowed criminal suspects to be sent for trial in mainland Chinese courts.
Opposition to the extradition bill has developed into wider concerns about the erosion of freedoms guaranteed under the "one country, two systems" formula put in place after Hong Kong's return to Chinese rule in 1997.
It was not yet clear whether the airport clashes had eroded the broad support the movement has so far attracted in Hong Kong, despite adding to the city's faltering economy.
The protests could push Hong Kong into a recession, research firm Capital Economics said, and risked "an even worse outcome if a further escalation triggers capital flight".
Hong Kong's property market, one of the world's most expensive, would be hit hard in that scenario, it added.
Financial Secretary Paul Chan unveiled a series of measures worth HK$19.1 billion ($2.44 billion) on Thursday to tackle economic headwinds, but he said it was not related to political pressure from the protests.
Business and citizens' groups have been posting full-page newspaper advertisements that denounce the violence and back Hong Kong's government.
The head of Macau casino operator Galaxy Entertainment , Lui Che-woo, urged talks to restore harmony. The protests have affected the neighbouring Chinese territory of Macau, with some visitors avoiding the world's biggest gambling hub amid transport disruptions and safety concerns. ($1 = 7.8401 Hong Kong dollars) (Additional reporting by Donny Kwok, Noah Sin, Kevin Liu and Twinnie Siu in HONG KONG, David Brunnstrom and Jonathan Landay in WASHINGTON, Mathieu Rosemain in PARIS, and David Ljunggren in OTTAWA; Writing by Farah Master; Editing by Paul Tait and Darren Schuettler)