China's military rotates new batch of troops into Hong Kong amid protests

China's military has rotated a new batch of troops into Hong Kong describing the move as routine, state media said on Thursday, as protests against Beijing continue to rock the Asian financial hub.

Asian and Western diplomats in Hong Kong watching the People's Liberation Army (PLA) movements had been expecting a routine rotation about this time and will be looking closely for any sign of increased numbers or unusual activity.

"The Hong Kong Garrison of the Chinese People's Liberation Army conducted the 22nd rotation of its members in the wee hours of Thursday since it began garrisoning Hong Kong in 1997," the official Xinhua news agency said.

"Approved by the Central Military Commission, the move is normal routine annual rotation in line with the Law of the People's Republic of China on Garrisoning the Hong Kong Special Administrative Region, which stipulates that 'the Hong Kong Garrison shall practice a system of rotation of its members'".

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Xinhua showed pictures of armed personnel carriers and trucks carrying troops at the border, though it was unclear from the caption if they were passing into or out of Hong Kong.

It also showed a picture of a small naval vessel arriving in Hong Kong.

Hong Kong has been engulfed in angry and sometimes violent protests against the government for three months, sparked by a now-suspended extradition bill and concerns that Beijing was trying to bring the territory under greater mainland control.

The protests are the greatest political threat to Hong Kong's government since the territory returned to Chinese rule in 1997, and one of the biggest popular challenges to Chinese leader Xi Jinping since he came to power in 2012.