China launched 39 aircraft, mostly fighter jets, near Taiwan Sunday — the largest show of force in months.

The launch came the same day the U.S. Navy sailed two aircraft carriers along with a pair of amphibious assault ships alongside allies in the Philippine Sea. 

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It's not clear what might have prompted the escalation, but China did something similar in early October during similar U.S. Navy operations.

Taiwan issued radio warnings and sent combat air patrol to deter the Chinese aircraft, while air defense missile systems were deployed to monitor them, Taiwan’s Ministry of National Defense said in the statement issued late on Sunday.

China claims Taiwan is part of its own territory and has rejected its claims of independence after the two countries split in 1949. The U.S. does not formally recognize Taiwan but maintains an unofficial relationship and is supportive of its democratic government. 

Chinese President Xi Jinping

Chinese President Xi Jinping speaks with Russian President Vladimir Putin via a video link, from the Great Hall of the People on December 2, 2019, in Beijing, China. (Photo by Noel Celis - Pool/Getty Images)

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Beijing has been increasing its presence near Taiwan and has been sending dozens of warplanes towards its defense zone, coinciding with calls by President Xi Jinping for Taiwan to be brought into China as part of a "peaceful reunification."

The U.S. military activities in the Philippine Sea coincide with its delivery of its first shipments of promised "lethal aid" to Ukraine on Friday, which included around 200,000 pounds of ammunition and weapons for frontline defenders. The U.S. Embassy in Kyiv hailed the shipment – delivered by airplane – as part of $2.7 billion of investments made in Ukraine.