The US has sent a warship on patrol within 12 nautical miles of artificial islands China has built in its territory in the South China Sea.
When the plan was first announced, the Chinese Embassy in Washington said the concept of freedom of navigation should not be used as an excuse for muscle-flexing.
Embassy spokesman Zhu Haiquan said the US should "refrain from saying or doing anything provocative, and act responsibly in maintaining regional peace and stability."
He said "freedom of navigation and overflight should not be used as excuse to flex muscle and undermine other countries\' sovereignty and security".
