Xinhua News Agency has reported that the People's Liberation Army (PLA) garrison in Hong Kong has just begun its 22nd troop rotation since they entered the SAR in 1997.
The report was released early Thursday morning after members of the public reported seeing army vehicles rolling down from the border with the mainland.
The official news agency said the rotation was based on the existing regulations for the Hong Kong garrison.
It said it was a routine action that had been approved by the Central Military Commission.
There were no further details, but eyewitnesses said they saw army trucks on the move in Shek Kong, Sha Tin and through the Eastern Harbour Tunnel.
The announcement comes amid growing fears of Central Government intervention in Hong Kong after more than two months of increasingly violent protests against the government's extradition bill and with anti-Beijing feeling riding high.
The Hong Kong garrison recently released a promotional video showing PLA troops in an anti-riot drill detaining "protesters".