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.

Chinese President Xi Jinping .

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.

China has long taken a carrot and stick approach towards Taiwan, both promising to work for peaceful "reunification" at the same time as threatening force. In the past year and a half China has staged two large-scale war games around the island.

Taiwan's foreign ministry, responding to the Biden-Xi meeting, said the government has never sought to predict whether or when China might attack but was concentrating on boosting its defenses and winning international support.

This is to "let China understand the high importance the international community attaches to peace and stability in the Taiwan Strait, the high price of starting a war, and to not act blindly without thinking," ministry spokesperson Jeff Liu said in Taipei.

Taiwan will neither provoke nor "advance rashly" but will also not succumb to pressure, Liu said.

Biden and Xi met for the first time in a year on Wednesday for talks aimed at easing friction between the two superpowers over military conflicts, drug-trafficking and artificial intelligence, and said they had made "real progress."

China has stepped up military activity to try and force democratically governed Taiwan to accept Beijing's sovereignty, despite strong objections from Taipei government, which says only the island's people can decide their future.

Senior U.S. military officers have said that Xi has ordered the People's Liberation Army to be prepared to invade Taiwan by 2027. Beijing has not ruled out using force to take the island, though it has never shared details about war preparations.