The US military said it sent two Navy warships through the Taiwan Strait on Sunday as the Pentagon increases the frequency of movement through the strategic waterway despite opposition from China.

The voyage risks further raising tensions with China, but will likely be viewed by self-ruled Taiwan as a sign of support from the Trump administration amid growing friction between Taipei and Beijing.

Taiwan is one of a growing number of flashpoints in the US-China relationship, which also include a trade war, US sanctions and China's increasingly muscular military posture in the South China Sea, where the United States also conducts freedom-of-navigation patrols.

The two destroyers were identified as the William P. Lawrence and Stethem. The 180-km wide Taiwan Strait separates Taiwan from China.

"The ships' transit through the Taiwan Strait demonstrates the US commitment to a free and open Indo-Pacific," the US Pacific Fleet said in a statement.

The United States has no formal ties with Taiwan, but is bound by law to help defend the island nation and is its main source of arms. The Pentagon says Washington has sold Taipei more than US$15 billion (RM62 billion) in weaponry since 2010.

China has been ramping up pressure to assert its sovereignty over the island, which it considers a wayward province of "one China" and sacred Chinese territory.

China said a recent Taiwan Strait passage by a French warship, first reported by Reuters on Wednesday, was "illegal".

Beijing's concerns about Taiwan are likely to factor strongly in this year's Chinese defence budget, following a stern New Year's speech from President Xi Jinping, threatening to attack Taiwan should it not accept Chinese rule.

China has repeatedly sent military aircraft and ships to circle the island on drills in the past few years and worked to isolate the island internationally, whittling down its few remaining diplomatic allies.

- Reuters