TOKYO — President Biden said Monday that America would intervene militarily if Beijing were to invade the Republic of China, saying the burden to protect Taiwan is “even stronger” after Russia’s invasion of Ukraine. It was one of the most forceful presidential statements in support of self-governing in decades, though the White House denied it marked any shift in American policy.

Mr. Biden, at a news conference in Tokyo, said “yes” when asked if he was willing to get involved militarily to defend Taiwan if China invaded. “That’s the commitment we made,” he added.

America traditionally has avoided making such an explicit security guarantee to Taiwan, with which it no longer has a mutual defense treaty, instead maintaining a policy of “strategic ambiguity” about how far it would be willing to go if China invaded. 

The 1979 Taiwan Relations Act, which has governed American relations with the island, does not require America to step in militarily to defend Taiwan if China invades, but makes it American policy to ensure Taiwan has the resources to defend itself and to prevent any unilateral change of status in Taiwan by Beijing.

Mr. Biden’s comments drew a sharp response from the mainland, which has claimed Taiwan to be a rogue province.

Chinese Foreign Ministry spokesman Wang Wenbin expressed “strong dissatisfaction and resolute opposition” to Mr. Biden’s comments. “China has no room for compromise or concessions on issues involving China’s core interests such as sovereignty and territorial integrity.”

He added, “China will take firm action to safeguard its sovereignty and security interests, and we will do what we say.”

A White House official said Mr. Biden’s comments did not reflect a policy shift.

Speaking alongside the Japanese prime minister, Fumio Kishida, Mr. Biden said any effort by China to use force against Taiwan would “just not be appropriate,” adding that it “will dislocate the entire region and be another action similar to what happened in Ukraine.”

China has stepped up its military provocations against democratic Taiwan in recent years aimed at intimidating it into accepting Beijing’s demands to unify with the communist mainland.

“They’re already flirting with danger right now by flying so close and all the maneuvers that are undertaken,” Mr. Biden said of China.

Under the “one China” policy, America recognizes Beijing as the government of China and doesn’t have diplomatic relations with Taiwan. However, America maintains unofficial contacts including a de facto embassy in Taipei, the capital, and supplies military equipment for the island’s defense.

Mr. Biden said it is his “expectation” that China would not try to seize Taiwan by force, but he said that assessment “depends upon just how strong the world makes clear that that kind of action is going to result in long-term disapprobation by the rest of the community.”

He added that deterring China from attacking Taiwan was one reason why it’s important that President Putin “pay a dear price for his barbarism in Ukraine,” lest China and other nations get the idea that such action is acceptable.

Fearing escalation with nuclear-armed Russia, Mr. Biden quickly ruled out putting American forces into direct conflict with Russia, but he has shipped billions of dollars in military assistance that has helped Ukraine put up a stiffer-than-expected resistance to Russia’s onslaught.

Taipei cheered Biden’s remarks, with a Ministry of Foreign Affairs spokeswoman, Joanne Ou, expressing “sincere welcome and gratitude” for the comments.

“The challenge posed by China to the security of the Taiwan Strait has drawn great concern in the international community,” said Ms. Ou. “Taiwan will continue to improve its self-defense capabilities, and deepen cooperation with the United States and Japan and other like-minded countries to jointly defend the security of the Taiwan Strait and the rules-based international order, while promoting peace, stability and prosperity in the Indo-Pacific region.”

It’s not the first time Biden has pledged to defend Taiwan against a Chinese attack, only for administration officials to later claim there had been no change to American policy. In a CNN town hall in October, Mr. Biden was asked about using the American military to defend Taiwan and replied, “Yes, we have a commitment to do that.”

Mr. Biden’s comments came just before he formally launched a long-anticipated Indo-Pacific trade pact that excludes Taiwan.

White House national security adviser Jake Sullivan confirmed Sunday that Taiwan isn’t among the governments signed up for the Indo-Pacific Economic Framework, which is meant to allow the U.S. to work more closely with key Asian economies on issues like supply chains, digital trade, clean energy and anticorruption.

Inclusion of Taiwan would have irked China.

Mr. Sullivan said America wants to deepen its economic partnership with Taiwan on a one-to-one basis.