Chinese President Xi Jinping told U.S. President Joe Biden during their four-hour meeting on Wednesday that Taiwan was the biggest, most dangerous issue in U.S.-China ties, a senior U.S. official told reporters.

The official quoted Xi as saying China’s preference was for peaceful “reunification” with the Chinese-claimed island of Taiwan, but that he went on to talk about conditions in which force could be used.

Xi was trying to indicate that China is not preparing for a massive invasion of Taiwan, but that does not change the U.S. approach, the official said.

“President Xi … underscored that this was the biggest, most potentially dangerous issue in U.S.-China relations, laid out clearly that, you know, their preference was for peaceful reunification but then moved immediately to conditions that the potential use of force could be utilized,” the senior U.S. official told reporters, referring to Xi’s comments on Taiwan.

Biden responded by assuring Xi that Washington was determined to maintain peace in the region.

“President Biden responded very clearly that the long-standing position of the United States was … determination to maintain peace and stability,” the official said.

“President Xi responded: look, peace is … all well and good but at some point we need to move towards resolution more generally,” the official said.