The United States is "deeply concerned" about Chinese paramilitary movement along the Hong Kong border, a US State Department spokeswoman says.
Another US official said Beijing had stationed large numbers of paramilitary People's Armed Police "near and further out from Hong Kong", in response to weeks of street protests in the territory.
The official said there was no sign they were moving towards the border.
The number of personnel was "in the thousands", said the official, who did not want to be identified.
"They have amped up training and made it all pretty visible," he said.
"There are no recent indicators that they are preparing to deploy."
China's state-run Global Times media outlet reported on Monday that People's Armed Police had been assembling in Shenzhen, a city bordering Hong Kong, "in advance of apparent large-scale exercises".
It cited video it had obtained showing numerous armoured personnel carriers, trucks and other vehicles on expressways heading in the direction of Shenzhen over the weekend.
It noted that the role of the PAP was "dealing with rebellions, riots, serious violent and illegal incidents, terrorist attacks and other social security incidents".
Satellite images made available to Reuters on Wednesday from Maxar Technologies showed dozens of vehicles, including what appeared to be armoured personnel carriers, at the Shenzhen Bay Sports Centre across the harbour from Hong Kong.
On Tuesday, US President Donald Trump cited American intelligence as saying that China was moving troops to its border with Hong Kong, and urged calm as clashes continued between protesters and authorities in the former British colony.
The State Department's expression of concern came after senior US lawmakers from both the Democratic and Republican parties called on Trump to take a tougher line with China as worries grew over a possible Chinese intervention.
The State Department spokeswoman 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" and for Beijing to adhere to its commitments to allow Hong Kong a high degree of autonomy.
She said the protests reflected "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," the spokeswoman said.
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 intervened directly to crack down on increasingly violent pro-democracy protests in the city.