China expressed concern yesterday after two U.S. Navy ships sailed through the Taiwan Strait, the third such operation this year.

"China will pay close attention to and stay informed of U.S. warships sailing through the Taiwan Strait from beginning to end, and has already expressed its concern to the U.S.," said Chinese foreign ministry official spokesman Geng Shuang at a regular press briefing in Beijing yesterday.

He urged the U.S .to respect the "One China Principle" to avoid "impairing China-U.S. relations and the peace and stability of the Taiwan Strait."

China still sees Taiwan as part of its territory to be reunified, despite the two sides being ruled separately since the end of a civil war on the mainland in 1949. That means Beijing views any ships passing through the straits as essentially a breach of its sovereignty.

At a monthly press briefing by China's defense ministry yesterday, official spokesman Ren Guoqiang said that the People's Liberation Army would maintain a "high degree of alert."

Lieutenant Rachel McMarr, a spokeswoman for the U.S. Pacific Fleet, said the destroyer USS Stockdale and oiler USNS Pecos sailed through the strait in a "routine" transit on Wednesday. "The ships' transit through the Taiwan Strait demonstrates the U.S. commitment to a free and open Indo-Pacific," McMarr said in a statement.

Beijing protested after the U.S. Navy sailed two warships through the Taiwan Strait on Oct. 22. A U.S. official told AFP that Chinese ships had asserted a "presence" during Wednesday's sailing, but all interactions between the two navies were "safe and professional."