Beijing is prepared to take military action to prevent any attempt to separate Taiwan from the mainland, Chinese Defense Minister Wei Fenghe has said.

According to Reuters and other news agencies, Wei also said on Thursday: “The Taiwan issue is related to China’s sovereignty and touches upon China’s core interests … on this issue, it is extremely dangerous to challenge China’s bottom line.”

That line would be if Taiwan, now known officially as the Republic of China, declared itself as the Republic of Taiwan.

Beijing’s relations with Taipei have been particularly strained since the island’s president, Tsai Ing-wen of the pro-independence Democratic Progressive Party, came to power in 2016.

Wei’s warning came three days after the United States sent two warships through the Taiwan Strait, the latest of gestures made by Washington in support of Taiwan, a democracy with an elected legislature and government.

Wei also touched on the situation in the South China Sea, where China’s claims are disputed by several neighboring countries. Wei said China opposes displays of strength and “provocations” from countries outside the region under the “pretense” of protecting freedom of flight and navigation.

The US routinely sends warships and military aircraft to the South China Sea to protect what it calls freedom of navigation in international waters.

If China were to try to take over Taiwan, it is likely, according to some sinologists, to be in 2021, the 100th anniversary of the founding of the ruling Communist Party of China.