Taiwan is an internal matter for China, Beijing's foreign ministry said on Monday, in response to President Donald Trump saying Chinese President Xi Jinping told him he will not invade the island while Trump is in office.

Trump made the comments in an interview with Fox News, ahead of talks in Alaska with Russian President Vladimir Putin over Moscow's war with Ukraine.

Asked about Trump's remarks at a daily news briefing in Beijing, Chinese Foreign Ministry spokesperson Mao Ning said Taiwan is an inalienable part of Chinese territory.

"The Taiwan issue is purely an internal affair of China, and how to resolve the Taiwan issue is a matter for the Chinese people," she said.

"We will do our utmost to strive for the prospect of peaceful reunification. But we will never allow anyone or any force to separate Taiwan from China in any way."

China views Taiwan as its own territory and has vowed to "reunify" with the democratic and separately governed island. Taiwan vehemently opposes China's sovereignty claims.

On Sunday, in its response to what Trump had said, Taiwan's foreign ministry said it "always closely monitors interactions between senior officials from the United States and China."

Taiwan will continue to work with countries that have "significant interests" in the Indo-Pacific region to maintain peace and stability in the Taiwan Strait, the ministry added.

The United States is Taiwan's most important international backer and arms supplier, despite the absence of formal diplomatic ties.

China, which has never renounced the use of force to bring the island under its control, regularly describes Taiwan as the most important and sensitive topic in its relations with the United States.