Twenty Chinese military aircraft entered Taiwan's air defense identification zone on Friday, in the largest incursion yet reported by the island's defense ministry and marking a dramatic escalation of tension across the Taiwan Strait.

The island's defense ministry said the air force deployed missiles to "monitor" the incursion into the southwestern part of its air defense identification zone. It also said its planes warned the Chinese aircraft, including by radio.

It marked the largest incursion to date by the Chinese air force since Taiwan's defense ministry began disclosing almost daily Chinese military flights over the waters between the southern part of Taiwan and the Taiwan-controlled Pratas Islands in the South China Sea last year.

Some of the Chinese aircraft flew in the airspace to the south of Taiwan and passed through the Bashi Channel which separates the island from the Philippines, Taiwan's defense ministry said in a statement.

A person familiar with Taiwan's security planning told Reuters the Chinese military was conducting exercises that would simulate an operation against U.S. warships that sail through the Bashi Channel.

China, which claims Taiwan as its own territory, has stepped up military activities near the democratic island in recent months, a move Taiwan says jeopardizes regional stability.

The presence of so many Chinese combat aircraft on Friday's mission – Taiwan said it was made up of four nuclear-capable H-6K bombers and 10 J-16 fighter jets, among others – was unusual and came as the island's air force suspended all training missions after two fighter jet crashes this week.

There was no immediate comment from China's defense ministry. Beijing routinely says such exercises are nothing unusual and are designed to show the country's determination to defend its sovereignty.

Earlier on Friday, Taiwan and the United States signed their first agreement under the administration of new president Joe Biden, establishing a Coast Guard Working Group to coordinate policy, after China's passing of a law that allows its coast guard to fire on foreign vessels.

While the United States, like most countries, has no formal diplomatic ties with Taiwan, it is bound by law to help Taiwan defend itself and is the island's main arms supplier.