Chinese officials described the flight of its bombers over the Sea of Japan as a routine training exercise, but Tokyo and Seoul believe the action was all about politics.

On Jan. 9 the Chinese squadron of six H-6 bombers, an intelligence-gathering aircraft and an early warning plane flew into the air defense identification zone of Japan over the Tsushima Strait, according to officials of the Joint Staff of the Defense Ministry.

While China is renowned for its many attempts to demonstrate military maritime advances, the latest incident shows Beijing is now intensifying its efforts in the sky.

Beijing has expressed its displeasure at Seoul's decision to deploy a ballistic missile defense system created by the United States.

The eight planes flew over the East China Sea, passed south of Tsushima island and continued on over the Sea of Japan before making a 360-degree turn to return home.

Air Self-Defense Force aircraft were scrambled because the Chinese planes entered Japan's ADIZ, but the Chinese squadron did not enter Japanese airspace.

It was believed to be the largest Chinese squadron to ever fly over the Tsushima Strait, which separates South Korea from Nagasaki Prefecture.

In January 2016, two Chinese planes flew over the strait, and another three were confirmed to have flown over it in August.

The flight was made in conjunction with Chinese naval vessels that were sailing in the Sea of Japan, said Liang Yang, a Chinese Navy spokesman, on Jan. 10.

"It was based on an annual training plan and was not targeting any specific nation," Liang said in a written statement. "The flight was made based on relevant international law."

He added that such joint training exercises would continue in the future.

On Jan. 5, two Chinese frigates and a supply ship entered the Sea of Japan after passing through the Tsushima Strait.

Those three ships passed back into the East China Sea from the Sea of Japan by going through the strait on Jan. 10, according to Defense Ministry officials.

The likely objective of the exercise was to improve flight capabilities of Chinese aircraft as well as gather intelligence about the Sea of Japan, said a diplomatic source in Beijing.

In its Jan. 10 edition, the Global Times, which is affiliated with the People's Daily, the Chinese Communist Party's mouthpiece, published an article that said the standard for training exercises over distant waters by Chinese naval aircraft had dramatically improved.

It included a comment from a military expert who said, "In order to pass over the narrow Tsushima Strait without intruding into Japanese airspace, there is a need for the planes to be equipped with a highly accurate satellite positioning system and the pilots must have advanced flying skills."

The Chinese planes also flew into South Korea's ADIZ as well as south of a submerged reef South Korea refers to as Ieodo Island. Both China and South Korea have claimed administrative rights over what is also known as the Socotra Rock, an area where the ADIZ of Japan, China and South Korea overlap.

South Korea and Japan have a system of alerting the other side before planes fly into overlapping ADIZs, according to officials of South Korea's Defense Ministry. However, South Korea does not have a similar setup with China, they added.

When the Chinese squadron entered South Korea's ADIZ, South Korean Air Force planes were scrambled. Pilots asked the Chinese pilots what the objective of their flight was and received the response of "training," ministry officials said.

"There was likely the aim of keeping cooperation between Japan, the United States and South Korea in check," said a high-ranking SDF officer.

A senior SDF officer added, "It could have been sending a message to (U.S. President-elect Donald) Trump, who is trying to deepen ties with Taiwan."

China has been highly critical of the ground-based Terminal High Altitude Area Defense system, which will be deployed by the U.S. military in South Korea.

(This article was written by Daisuke Nishimura in Beijing, Yoshihiro Makino in Seoul and Yusuke Fukui in Tokyo.)