Xi Jinping told Donald Trump during a phone call on Monday that Taiwan’s “return to China” is a key part of the post-war international order, state news agency Xinhua reported.

“China and the United States once fought side by side against fascism and militarism, and should now work together to safeguard the outcomes of World War Two,” Xi was quoted as saying by Xinhua.

China regards Taiwan as part of its territory and has not ruled out the use of force to take control of it, though the island’s government rejects Beijing’s claim and says only Taiwan’s people can decide their future.