The White House is monitoring the sudden "congregation" of Chinese forces at the border with Hong Kong, according to reports, following another night of unrest and clashes between protesters and police.

A senior US official who briefed reporters on the condition of anonymity said a number of units had gathered, but it's unclear if they are security police or part of China's military, Bloomberg reports.

It comes as Beijing accused the United States of inciting the increasingly unruly protests in Hong Kong, which began two months ago over a proposed extradition bill that could see citizens sent to the mainland.

Since then, demonstrations have evolved into a statement against Beijing's influence in the operation of the relatively autonomous region.

Bloomberg cites the White House official as saying the US is watching China's mainland border manoeuvres. Reports of the gathering of forces has sparked panic among Hong Kong locals on social media.