HONG KONG (REUTERS) - Hong Kong's Airport Authority said normal flight operations would resume on Thursday (Aug 15) after pro-democracy protests forced the cancellation of nearly 1,000 flights this week, while the city braced for more mass protests through the weekend.

China reiterated on Wednesday that Hong Kong's protest movement was "near terrorism", and more street clashes followed ugly and chaotic scenes at the airport on Tuesday, when protesters set upon two men they suspected of being government sympathisers.

Police and protesters faced off again on the streets of the financial hub overnight, with riot officers quickly firing tear gas as their response to demonstrators toughens.

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.

Heightened security would remain at the city's international airport and the Hong Kong Airport Authority said late on Wednesday that an application for protests to be held in the terminal must be made in advance with a "Letter of No Objection" to be obtained from police.

More protests are planned on Friday and over the weekend in different areas of the Chinese-controlled territory.

Protesters have expressed remorse after a peaceful sit-in turned violent at one of the world's busiest airports earlier this week.

It was not clear whether the violent clashes might have eroded the broad support the movement has so far attracted in Hong Kong. The protests have also hit the city's faltering economy.

The United States said it was deeply concerned at news of Chinese police forces gathering near the border, urged Hong Kong's government to respect freedom of speech, and issued a travel advisory urging caution when visiting the city.