WASHINGTON (Reuters) - The United States sent two Navy ships through the Taiwan Strait on Wednesday in the third such operation this year, as the U.S. military increases the frequency of transits through the strategic waterway despite opposition from China.

The U.S. Navy Arleigh Burke-class guided-missile destroyer USS Stockdale transits the Philippine Sea November 16, 2018. Picture taken November 16, 2018. U.S. Navy/Mass Communication Specialist 3rd Class Connor D. Loessin/Handout via REUTERS

The voyage risks further raising tension with China but will likely be viewed in self-ruled Taiwan as a sign of support from U.S. President Donald Trump’s government amid growing friction between Taipei and Beijing.

“The ships’ transit through the Taiwan Strait demonstrates the U.S. commitment to a free and open Indo-Pacific,” U.S. Pacific Fleet said in a statement.

“The U.S. Navy will continue to fly, sail and operate anywhere international law allows.”

It said the passage was carried out by the destroyer USS Stockdale and the Pecos, a replenishment vessel.

Taiwan’s defense ministry said it was a normal transit through international waters in the Taiwan Strait and Taiwan forces had monitored the passage of the ships.

China’s foreign ministry spokesman Geng Shuang said at a regular briefing on Thursday that Beijing had expressed its concern over the passage to the United States.

“We urge the United States to ... cautiously and appropriately handle the Taiwan issue, avoid damaging the peace and stability of the Taiwan Strait and China-U.S. relations,” he said.

‘CORE ISSUE’

China’s defense ministry spokesman, Ren Guoqiang, said their military was fully aware of the U.S. action.

“The Taiwan issue is a matter of China’s sovereignty and territorial integrity, and is the most important, most sensitive core issue in China-U.S. relations, and involves China’s core interests,” Ren told a monthly news briefing.

“The will and determination of the Chinese military to protect our sovereignty and territorial integrity is unswerving.”

The U.S. patrol comes ahead of an expected meeting between Trump and Chinese President Xi Jinping this week in Argentina on the sidelines of a G20 summit.

The U.S. Navy conducted a similar mission in the strait’s international waters in July, which had been the first such voyage in about a year.

The latest operation shows the U.S. Navy is increasing the pace of strait passages.

Washington has no formal ties with Taiwan, but is bound by law to help it defend itself and is the island’s main source of arms. The Pentagon says Washington has sold Taiwan more than $15 billion in weaponry since 2010.

China has been ramping up pressure to assert its sovereignty over the island, which it considers a breakaway province of “one China”.

Over the weekend, Taiwan’s ruling pro-independence Democratic Progressive Party suffered heavy losses in mayoral and county elections to the China-friendly Kuomintang, which has been welcomed by Beijing.

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