The head of China’s defense and President Joe Biden’s National Security Adviser Jake Sullivan met Thursday and sparred over the U.S.’ ongoing relationship with Taiwan.

Sullivan traveled to China this week for high-level strategic talks with Beijing officials, including Chinese President Xi Jinping. During a meeting with Chinese General of the People’s Liberation Army Zhang Youxia on Thursday, Youxia warned Sullivan against the U.S.’ support for Taiwan and demanded that it stop offering the island military assistance, according to a readout from the Chinese Ministry of National Defense.

Youxia “stressed that the Taiwan question is at the very core of China’s core interests… and the first red line that must not be crossed in China-U.S. relations,” according to a readout of the meeting. “China is committed to safeguarding peace and stability in the Taiwan Strait, yet ‘Taiwan independence’ forces are as incompatible with peace and stability of the Taiwan Strait as fire with water.”

China’s view is that Taiwan is a rogue state that needs to be brought back into the fold, whether peacefully or by force. The country has expressed anger in the past when other nations help or maintain relations with Taiwan.

“The [Chinese military] will definitely take countermeasures against the provocations of the ‘Taiwan independence’ forces,” Youxia told Sullivan, according to the readout. “China urges the U.S. side to stop military collusion with Taiwan, stop arming Taiwan, and stop spreading false narratives on Taiwan.”

Sullivan asserted “that both countries have a responsibility to prevent competition from veering into conflict or confrontation,” according to a separate readout of the meeting provided by the White House. Sullivan also “raised the importance of cross-Strait peace and stability [and] the U.S. commitment to freedom of navigation in the South China Sea,” although Taiwan is not directly mentioned in the readout.

Sullivan’s meeting with Xi appeared to focus on different matters, such as prioritizing communication and building on commitments made between the Chinese president and Biden in November, according to a readout of the meeting from the White House. Biden is expected to call Xi sometime in the coming weeks, according to a White House readout; it is unclear whether they will meet in person again before Biden’s term ends in January.

The Biden administration has faced criticisms for its handling of China’s increasing aggression against U.S. allies and the West. China has expanded its military operations in the Asia-Pacific region and has been suspected of attempting to undermine American society through a variety of methods in recent years, including through cyber attacks against critical U.S. infrastructure systems, stealing U.S. technologies and allowing the flow of fentanyl precursors from its mainland into South and Central America, where it is turned into a final product and shipped over the U.S. southern border.