China's Hong Kong liaison office said Wednesday that anti-government protesters were no different from "terrorists," and U.S. President Donald Trump said Chinese troops were moving to the border with Hong Kong and urged calm.

The Beijing-based Hong Kong and Macao Affairs office Wednesday said extremely violent crimes must be severely punished in accordance with the law.

The strongly worded statements by China's central government follows violent clashes between black-clad protesters and riot police at Hong Kong's international airport, which had hundreds of flights halted for a second day.