China says warned US warship as it transited Taiwan Strait

China's military said on Wednesday that it had monitored and warned a US warship that had sailed through the sensitive Taiwan Strait, a mission that happened shortly after China carried out drills near the island.

The US Navy's 7th Fleet said the guided-missile cruiser USS Port Royal conducted a "routine" Taiwan Strait transit through international waters "in accordance with international law" on Tuesday, the second such mission in two weeks.

The United States has been carrying out such voyages about once a month, angering China, which views them as a sign of support for Taiwan, the democratically governed island that Beijing views as Chinese territory.

The People's Liberation Army's Eastern Theatre Command said in a statement that its forces had monitored the ship throughout and "warned" it.

"The United States frequently stages such dramas and provokes trouble, sending wrong signals to Taiwan independence forces, and deliberately intensifying tensions across the Taiwan Strait," it added.

"Theatre troops maintain high alert at all times, resolutely counteract all threats and provocations, and resolutely defend national sovereignty and territorial integrity."

The US Navy said the ship "transited through a corridor in the Strait that is beyond the territorial sea of any coastal State".

"Port Royal's transit through the Taiwan Strait demonstrates the United States' commitment to a free and open Indo-Pacific. The United States military flies, sails, and operates anywhere international law allows."

Taiwan's Defence Ministry said the US ship sailed north through the strait, and that the situation in the waterway was "as normal".

Late on Tuesday, the ministry said a single Chinese WZ-10 attack helicopter had briefly crossed the strait's unofficial mid line, which combat aircraft from both sides normally avoid passing through, though China's air force does on occasion do so.

It also reported that two Chinese KA-28 anti-submarine helicopters were spotted in an area roughly halfway between Taiwan's southwestern coast and the Taiwan-controlled Pratas Islands at the top part of the South China Sea.

China's armed forces carried out another round of exercises near Taiwan last week to improve joint combat operations, the People's Liberation Army said on Monday, after the Chinese-claimed island reported a spike in activity. 

The guided-missile destroyer USS Sampson sailed through the Taiwan Strait on 27 April, which China condemned, saying such missions "deliberately" harm peace and stability. 

The United State, like most countries, has no formal diplomatic ties with Taiwan but is its most important international supporter and arms supplier, making it a constant source of tension between Beijing and Washington.