Chinese leader Xi Jinping warned U.S. President Donald Trump to be “prudent” about arms sales to Taiwan during their first call since November, emphasizing that Taiwan is the most important issue in China-U.S. relations.

Xi stressed that China will safeguard its sovereignty and territorial integrity and will not allow Taiwan to separate.

Taiwan’s President Lai Ching-te responded by saying ties with the U.S. are “rock solid” and cooperation will continue uninterrupted.

The call comes after the U.S. proposed its largest-ever arms package to Taiwan, valued at over $11.1 billion, prompting Chinese military drills and domestic debate in Taiwan over defense spending, The Guardian has reported.

Trump also discussed other topics with Xi, including Russia’s war in Ukraine, Iran, U.S.-China energy trade, and a planned visit to China in April.