HONG KONG — China sent 39 warplanes into Taiwan's air defense identification zone (ADIZ) on Sunday, Taiwan's Defense Ministry said, the largest such incursion this year.

The flights by the People's Liberation Army aircraft came a day after the United States and Japanese navies put on a massive show of force in the Philippine Sea, putting together a flotilla that included two US Navy aircraft carriers, two US amphibious assault ships and a Japanese helicopter destroyer, essentially a small aircraft carrier.

Two US guided-missile cruisers and five destroyers were also part of the exercise. The Philippine Sea is the area of the Pacific Ocean east of Taiwan, between the self-ruled island and the US territories of Guam and the Northern Mariana Islands. The Navy did not say how close the flotilla was to Taiwan.

"Freedom at its finest! Nothing reaffirms our commitment to a #FreeandOpenIndoPacific like 2 Carrier Strike Groups, 2 Amphibious Ready Groups sailing alongside our close friends from the Japan Maritime Self Defense Force," Vice Adm. Karl Thomas, commander of the US 7th Fleet based in Japan, said in a tweet.

A US Navy statement said the mass of warships was "conducting training to preserve and protect a free and open Indo-Pacific region."

Taiwan and mainland China have been governed separately since the defeated Nationalists retreated to the island at the end of the Chinese civil war more than 70 years ago.

But China's ruling Chinese Communist Party (CCP) views the self-ruled island as part of its territory -- despite having never controlled it.

Beijing has not ruled out military force to take Taiwan and has kept pressure on the democratic island over the past few years with frequent warplane flights into Taiwan's ADIZ. The US Federal Aviation Administration defines an ADIZ as "a designated area of airspace over land or water within which a country requires the immediate and positive identification, location and air traffic control of aircraft in the interest of the country's national security."

Sunday's incursions were made by 24 J-16 fighter jets, 10 J-10 fighter jets, two Y-9 transport aircraft, two Y-8 anti-submarine warning aircraft, and one nuclear-capable H-6 bomber, the Taiwanese Defense Ministry said in a statement on Sunday.

In response, the Taiwanese military issued radio warnings and deployed air defense missile systems to monitor the activities, it added.

The incursions on Sunday marked the highest daily number of Chinese warplanes entering Taiwan's ADIZ this year. The highest number of incursions ever recorded was on October 4 last year, when 56 military planes flew into the area on the same day.

While the Chinese incursions Sunday were likely a reaction to the large naval presence Tokyo and Washington were putting in the area, they also served another purpose, said Collin Koh, research fellow at the S. Rajaratnam School of International Studies in Singapore.

"No doubt this is part of the broader campaign by Beijing aimed at eroding the will and ability of Taiwan to continue resisting," Koh said.

He pointed to the recent crash of one of Taiwan's best fighter planes, an F-16V, and the toll its taking on the island's air force to respond to persistent PLA incursions into Taiwan's defense zone.

"Certain politicians and retired military officers (in Taiwan) have raised the issue of possible pilot shortage and insufficient training in the face of operational requirements in responding to frequent PLA flybys," Koh said.

The crash and those statements "would potentially sow concerns amongst the public regarding the island's ability to stand its ground against the mainland's repeated and avowedly increasing military provocations," especially as Beijing has vowed to continue the incursions, he said.

"The latest major flyby, while obviously targeted at the allied show of force in the Philippine Sea, will definitely have some intended reinforcement effect on the ongoing debates in Taiwan," Koh said.

Carl Schuster, a former director of operations at the US Pacific Command's Joint Intelligence Center, said China is trying to keep Taiwan off balance and tired -- "like a tennis player pushes his opponent to chase the ball across the court" -- sending larger formations of aircraft toward the island from more distant bases on the mainland.

He also said the US-Japan naval exercises sent a message to China not only about Taiwan, but about Beijing's actions in the South China Sea and near the Japan-controlled Senkaku Islands, which China calls the Diaoyus and claims, like Taiwan, are part of its sovereign territory.

The islands are closer to Taiwan than Tokyo, and Chinese ships have been an almost constant presence near them for months, Japan's defense minister told CNN last fall.

The US says the Senkakus fall under the US-Japan mutual defense treaty, which obligates Washington to defend them like any other part of Japanese territory.

Schuster said while the large naval exercise sends a message, its location meant it wasn't overly provocative.

The Philippine Sea lies outside what is called the first island chain, the waters within which are largely claimed by China. Schuster said keeping the US-Japan naval exercises outside the chain showed there was no threat to the Chinese mainland. — CNN