Japan and the United States conducted a joint exercise involving fighter jets over the Sea of Japan, the Defense Ministry said Thursday, a day after North Korea test-fired three ballistic missiles.

The flight of four F-15s from the Japan Air Self-Defense Force's Chitose base in Hokkaido and four F-16s from the U.S. Air Force base in Misawa, Aomori Prefecture, on Wednesday was meant to confirm the two forces' "readiness posture," the ministry said.

Japanese and U.S. fighter jets conduct a joint exercise above the Sea of Japan on May 25, 2022. (Photo courtesy of the Defense Ministry)(Kyodo)

The U.S. military said in a news release the drill was intended to "showcase combined capabilities to deter and counter regional threats."

After U.S. President Joe Biden concluded his first trip to South Korea and Japan, North Korea went ahead with yet another round of missile tests early Wednesday morning.

One of the three missiles fired by North Korea off its east coast was an intercontinental ballistic missile, according to the South Korean government.

The ballistic missile launches, already the 15th since the beginning of this year, came as Japan, South Korea and the United States have been bracing for a possible nuclear test by North Korea.

The joint flight exercise was held also after China and Russia flew six strategic bombers in total over the Sea of Japan, the East China Sea and the Pacific on Tuesday, when Tokyo was hosting a summit of the so-called Quad group with the United States, Australia and India.

The leaders of the four countries agreed during the summit to promote a free and open Indo-Pacific, a vision widely regarded as aimed at keeping China's expanding regional ambitions in check.

Some parallels have been drawn between Russia's invasion of Ukraine and China's increasing pressure on Taiwan and the two countries have conducted a number of military exercises around Japan.

Following the bomber flight, Japan conveyed its "grave concern" to China and Russia, according to Chief Cabinet Secretary Hirokazu Matsuno.

Russian Foreign Ministry spokeswoman Maria Zakharova criticized Japan's protest as "groundless and absurd" in a statement released Wednesday, warning Moscow will boost its military capabilities.

Zakharova said the joint warplane flight was a "patrol" that Russia and China have regularly carried out based on international law, labeling it as conducive to peace and stability in the Asia-Pacific region.

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