North Korea on Saturday conducted a fourth round of ballistic missile tests in one week, the country's most ever, launching two of them toward the sea off its east coast, the Japanese government said, in an apparent protest against joint naval drills held this week involving the United States and South Korea.

North Korea had already fired ballistic missiles toward the Sea of Japan on three occasions since last Sunday, in defiance of U.N. Security Council resolutions, driving a record-breaking pace of such testing this year.

The two short-range ballistic missiles were fired from the Sunan area of Pyongyang between 6:45 a.m. and 7:03 a.m., according to the South Korean military. The fourth test came a day after Japan joined the United States and South Korea for anti-submarine drills.

The missiles traveled about 350 to 400 kilometers with a maximum altitude of 50 km before apparently falling into waters off the country's east coast and outside Japan's exclusive economic zone, Toshiro Ino, Japanese senior vice defense minister, told reporters.

They may have flown on irregular trajectories, Ino said without elaborating. Japan lodged a protest with North Korea via the Japanese Embassy in Beijing.

"It threatens the peace and security of Japan, this region and the international community, and is absolutely unacceptable," Ino said.

In South Korea, President Yoon Suk Yeol said in a speech delivered at a military ceremony that "North Korea has not abandoned its obsession with nuclear weapons and missiles," adding Seoul will strengthen its alliance with the United States to counter Pyongyang's provocations.

The United States and South Korea held joint military exercises in the Sea of Japan between Monday and Thursday for the first time in about five years.

The South Korean military said the missiles had a maximum altitude of about 30 km, flying 350 km with a maximum speed of Mach 6. The latest round launched by North Korea brought the total number this year, including cruise missiles, to 22.

The U.S. Indo-Pacific Command said it had assessed that the latest missile launch did not pose an immediate threat to U.S. territory or its allies but highlighted the "destabilizing impact" of North Korea's "unlawful" weapons programs.

The U.S. commitments to the defense of Japan and South Korea remain "ironclad," the command added.

U.S. Vice President Kamala Harris traveled to the Demilitarized Zone on the border of the two Koreas on Thursday, showing Washington's commitment to the defense of South Korea.

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