The Chinese foreign ministry said on Monday that China has lodged “stern representations” with the United States, after U.S. President Joe Biden said U.S. forces would defend Taiwan in the event of a Chinese invasion.

China reserves the right to take all necessary measures in response to activities that split the nation apart, said Mao Ning, spokesperson at the foreign ministry, at a regular media briefing.

“We are willing to do our best to strive for peaceful reunification. At the same time, we will not tolerate any activities aimed at secession,” Mao said.

She also urged the U.S. to handle Taiwan-related issues “carefully and properly”, and not send “wrong signals” to Taiwan independence separatist forces, warning the United States not to seriously damage Sino-U.S. relations and the peace in the Taiwan Strait.

“There is only one China in the world, Taiwan is part of China, and the government of the People’s Republic of China is the only legitimate government of China,” said Mao.