The destroyers USS Mustin and USS Benfold sailed into the narrow waterway separating Taiwan and China on Saturday morning and were expected to continue on a northeast course, Taiwan's defence ministry said in a statement.

"The military is monitoring the situation in neighbouring areas, and has the confidence and abilities to maintain regional stability and defend national security," the statement added.

A defence ministry official told AFP the ships were still in the strait on Saturday night, sailing in what he described as international waters.

The warships' entry into the strait comes as Washington and Beijing are locked in a trade war and as tensions escalate between Beijing and Taipei.

China sees self-ruling democratic Taiwan as part of its territory to be reunified, but the island sees itself as a sovereign country.

Although the US does not have official diplomatic relations with Taiwan, it is its most powerful ally and biggest arms supplier.

China has stepped up diplomatic and military pressure on Taiwan since Beijing-sceptic President Tsai Ing-wen took office two years ago as her government refuses to acknowledge that the island is part of "one China."

Relations between the US and Taiwan have warmed recently as President Donald Trump signed legislation paving the way for mutual visits by top officials and the US government approved a licence required to sell submarine technology to Taiwan.