At a daily Foreign Ministry press briefing on Thursday, spokesperson Hua Chunying said Beijing “firmly opposes and strongly condemns this move by the U.S.,” in which a military C-40A transport aircraft entered Taiwanese airspace on Tuesday with permission, but did not land.

Hua said the flight “gravely violates international law and basic norms governing international relations.”

China does not consider Taiwan—officially called the Republic of China—to be an independent country, but rather a wayward province of a unified country. Under its “One China” policy, Beijing has vowed to bring the democratic island back under its control, whether by diplomatic means or by force.

Taiwan has long been a diplomatic flashpoint between the U.S. and China. Though Washington does not officially recognize Taiwan as an independent nation, U.S. lawmakers and leaders have maintained bipartisan support for its self-determination. This includes significant military sales to the island to help it guard against a potential Chinese invasion.

Beijing regularly protests such sales, such as the recent deal to send some $180 million worth of advanced U.S.-made torpedoes to Taiwan.

“We urge the U.S. to adhere to the one-China principle and the three China-U.S. joint communiques, and immediately stop such unlawful and provocative behaviors,” Hua said Thursday. “China will take all necessary measures to safeguard national sovereignty, security and development interests.”

Soon after the U.S. flight over the island, several Chinese Su-30 fighter jets entered Taiwanese airspace. Taiwan’s defense ministry said it scrambled several jets to intercept the Chinese aircraft and “broadcast warnings” notifying the pilots that they had entered Taiwan’s air defense identification zone.

Relations between China and Taiwan have deteriorated in recent months amid Beijing’s curtailment of human rights in Hong Kong, the coronavirus pandemic and the Taiwanese presidential election, in which Beijing-skeptic President Tsai Ing-wen—the leader of the liberal and nationalist Democratic Progressive Party—was re-elected for a second term.

Tsai and her allies have accused Beijing of waging a disinformation and meddling campaign in Taiwan seeking to undermine the democratic process. China has also continued its efforts to peel away Taiwan’s remaining diplomatic allies and neuter the island on the international stage.

Meanwhile, Chinese officials have stressed that military force remains an option for bringing Taiwan back into the “One China” fold.

Last month, General Li Zuocheng—the chief of the Joint Staff Department and member of the Central Military Commission—said: “If the possibility for peaceful reunification is lost, the people’s armed forces will, with the whole nation, including the people of Taiwan, take all necessary steps to resolutely smash any separatist plots or actions.”