Beijing said it is on "heightened alert" after the US Navy sailed two ships through the Taiwan Strait on Wednesday in the third such operation this year.

Chinese defence ministry spokesman Ren Guoqiang also said Beijing would "resolutely protect China's sovereignty" in relation to Taiwan, and that the Chinese military was "clear and fully aware of" the movement of the US ships.

The voyage by the guided missile destroyer USS Stockdale and the USNS Pecos, an oiler, came ahead of an expected meeting between Chinese President Xi Jinping and US President Donald Trump at the G20 summit this week in Argentina, with hopes of easing trade and military tensions between the two nations.

Chinese Navy training. Photo / 123RF
Chinese Navy training. Photo / 123RF

The US move also came after Taiwan's ruling, pro-independence Democratic Progressive Party suffered heavy losses in mayoral and county elections to the Beijing-friendly Kuomintang.

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"The People's Liberation Army will continue to maintain heightened alert and resolutely protect China's sovereignty and territorial integrity," Ren said on Thursday.

Beijing sees the self-ruled Taiwan as a breakaway province that must eventually be reunified with the mainland, by force if necessary.

In talks this month in Washington with US Secretary of State Mike Pompeo and US Defence Secretary James Mattis, Chinese officials reiterated their opposition to US support for Taiwan.

Ren repeated Beijing's call for Washington to abide by the one-China principle, so as to avoid undermining China-US relations and regional stability.

Separately, Chinese foreign ministry spokesman Geng Shuang said on Thursday that China had "expressed concerns" to the US.

"The issue over Taiwan is the most important and most sensitive in Sino-US relations. We hope the US side can abide by the one-China principle and the three communiques, and to exercise caution when it is handling the Taiwan issue," he said.

The US Navy conducted similar missions through the Taiwan Strait in July and October after a year-long hiatus, drawing protests from Beijing which sees such operations as an affront to Chinese sovereignty.

Taiwan Air Force IDF Fighter Jets fly over the drill area during a military drill. Photo / Getty Images
Taiwan Air Force IDF Fighter Jets fly over the drill area during a military drill. Photo / Getty Images

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

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"The US Navy will continue to fly, sail and operate anywhere international law allows," it said.

With Xi and Trump set to meet at the end of the week, Taiwan is a growing source of friction.

Last month, Xi told the military region responsible for monitoring Taiwan and the South China Sea to "prepare for war".

On the same day, Chinese Defence Minister Wei Fenghe vowed that Beijing would not cede "a single inch" of its territory.

- South China Morning Post