Sullivan arrived in Beijing on Tuesday, the first US national security advisor to visit China since 2016, for three days of talks with Foreign Minister Wang Yi and other high-ranking officials.

The visit came as China became embroiled in security rows with US allies Japan and the Philippines.

On Thursday morning, Sullivan met with senior Chinese army chief Zhang Youxia at the Beijing headquarters of the Central Military Commission.

"It's rare that we have the opportunity to have this kind of exchange," Sullivan told Zhang in opening remarks.

The two officials agreed to hold a call between the two sides' theatre commanders "in the near future", a readout from the White House added.

Sullivan also raised the importance of "freedom of navigation" in the South China Sea, where Beijing and Manila have clashed in recent months, and "stability" in the Taiwan Strait, Washington said.

Zhang, in turn, warned that the status of the self-ruled island was "the first red line that cannot be crossed in China-US relations".

"China has always been committed to maintaining peace and stability in the Taiwan Strait," he said, according to a readout by Beijing's defence ministry.

"But 'Taiwan independence' and peace and stability in the Taiwan Strait are incompatible," he said.

"China demands that the US halts military collusion with Taiwan, ceases arming Taiwan, and stops spreading false narratives related to Taiwan," Zhang added.

- 'Destabilizing actions'-

Thursday's talks also saw Sullivan express "concerns about (Chinese) support for Russia's defense industrial base", the readout added -- echoing longstanding US claims that Beijing has rejected.

He also raised "the need to avoid miscalculation and escalation in cyber space, and ongoing efforts to reach a ceasefire and hostage deal in Gaza", the White House said.

On Wednesday, Sullivan and Wang discussed plans for their leaders to talk in the coming weeks -- and clashed over China's increasingly assertive approach in disputed maritime regions.

Sullivan "reaffirmed the United States' commitment to defending its Indo-Pacific allies", the White House said.

He also "expressed concern about (China's) destabilizing actions against lawful Philippine maritime operations" in the disputed South China Sea, it said.

Chinese state media reported that Wang issued his own warning to Washington.

"The United States must not use bilateral treaties as an excuse to undermine China's sovereignty and territorial integrity, nor should it support or condone the Philippines' actions of infringement," Wang told Sullivan, according to state broadcaster CCTV.

Wang and Sullivan previously met five times over the past year and a half -- in Washington, Vienna, Malta and Bangkok, as well as alongside US President Joe Biden and Chinese leader Xi Jinping in Woodside, California in November 2023.

During their latest encounter, they also discussed the tense issue of Taiwan, the self-governing democracy that China claims.

China has kept up its sabre-rattling since the inauguration this year of President Lai Ching-te, whose party emphasises Taiwan's separate identity.