China said Thursday its troops were "on constant high alert" after US and Canadian warships passed through the Taiwan Strait, their second joint passage in two months.

The USS Rafael Peralta, an Arleigh Burke-class destroyer, and the Royal Canadian Navy's Halifax-class frigate HMCS Ottawa conducted a "routine" transit through the strait on Wednesday, the US Navy said.

"Troops in the theatre remain on constant high alert, and will resolutely protect national sovereignty and security as well as regional peace and stability," Senior Colonel Shi Yi, a spokesman for China's Eastern Theatre Command, said in a statement.

Shi slammed the latest transit as a "public hype" and said its naval and air forces had "trailed their entire course."

The ships' passage comes after two warships belonging to the US and Canada sailed through the narrow waterway separating the island and mainland China on September 9.

The United States and its Western allies have increased "freedom of navigation" crossings by naval vessels both of the Taiwan Strait and the disputed South China Sea, to reinforce that both are international waterways, angering Beijing.

The Seventh Fleet said in a statement the transit had been made in accordance with international law and "through a corridor in the Strait that is beyond the territorial sea of any coastal State".

"Cooperation like this represents the centerpiece of our approach to a secure and prosperous region where aircraft and ships of all nations may fly, sail, and operate anywhere international law allows."  

Taiwan's defense ministry said Thursday it was monitoring the passage on Wednesday night, but added that "the situation was normal".

China, which claims Taiwan as its own territory to be seized one day, has stepped up military and political pressure on the self-ruled island as relations have plunged in recent years.

In September, China sent in 103 planes around Taiwan within a 24-hour period, which Taipei described as "a recent high".