China expressed strong dissatisfaction and firm opposition on Monday over US warships entering waters around islands and reefs in the South China Sea.

Foreign Ministry spokesman Geng Shuang said at a news conference that two US warships entered the Nanxun Reefs and Chigua Reefs areas of the Nansha Islands without the permission of the Chinese government.

The moves violated China's sovereignty and undermined the peace, security and good order of the sea areas, Geng said.

"The situation in the South China Sea has been stabilizing steadily under the joint efforts of China and the Association of Southeast Asian Nations. China urges the US to stop such provocative actions and respect China's sovereignty, security interests and the efforts that we have made to maintain the peace and stability in the South China Sea," he said.

On Monday, two US guided-missile destroyers, Preble and Chung Hoon, sailed into waters within 12 nautical miles of the Nanxun Reef and Chigua Reef in the Nansha Islands.

Senior Colonel Li Huamin, spokesman for the People's Liberation Army Southern Theater Command, said the Navy identified the ships and warned them to leave.

China holds indisputable sovereignty over the South China Sea islands, and no provocative moves by a foreign force can change that, Li said.

The command will continue to stay on high alert and take all necessary measures to resolutely defend China's sovereignty and safeguard peace and stability in the South China Sea, he said.

Yuan Zheng, an expert at the Institute of American Studies of the Chinese Academy of Social Sciences, said the US has used freedom of maritime navigation as an excuse to exert its power at sea.

"The influence of the US on Southeast Asia is actually declining, so it won't let go on this issue and takes it as something to show its existence," he said.

Yuan said such moves will definitely have a negative impact and will add uncertainty to the South China Sea.