A protester is overcome by tear gas near the Shum Shui Po police station in Hong Kong on Wednesday. Photo: AP Photo/Vincent Yu

Washington – The United States is deeply concerned about Chinese paramilitary movement along the Hong Kong border, a US State Department spokesperson said on Wednesday, while warning that continued erosion of the territory's autonomy put at risk its special status in international affairs.

The official reiterated a US call for all sides to refrain from violence and said it was important for the Hong Kong government to respect "freedoms of speech and peaceful assembly".

"The United States is deeply concerned by reports of Chinese paramilitary movement along the Hong Kong border," the spokesperson said. 

"The United States strongly urges Beijing to adhere to its commitments… to allow Hong Kong to exercise a high degree of autonomy.

"We condemn violence and urge all sides to exercise restraint, but remain staunch in our support for freedom of expression and freedom of peaceful assembly in Hong Kong," the spokesperson said.

"The ongoing demonstrations in Hong Kong reflect the sentiment of Hongkongers and their broad and legitimate concerns about the erosion of Hong Kong’s autonomy.

"The continued erosion of Hong Kong’s autonomy puts at risk its long-established special status in international affairs."

A 1992 US law affords Hong Kong preferential treatment in matters of trade and economics compared with China. Areas of special treatment include visas, law enforcement and investment.

A prominent US senator warned China on Tuesday that Hong Kong could lose its special US trade status if Beijing intervenes directly to crack down on increasingly violent pro-democracy protests in the city.

Earlier on Wednesday, the State Department issued a travel advisory urging "increased caution in Hong Kong due to civil unrest".

Hong Kong has been engulfed in protests since earlier this summer. Flights resumed on Wednesday at the Hong Kong airport, one of the world's busiest, which shut down for two days after demonstrators occupied it.