SEOUL, SOUTH KOREA -- The U.S. aircraft carrier Ronald Reagan concluded a naval exercise with South Korea in the Philippine Sea on Saturday, the two militaries said, amid signs that North Korea is possibly preparing to conduct its first nuclear test explosion since 2017.

The three-day exercise that began Thursday in international waters off the Japanese island of Okinawa was apparently the allies' first joint drill involving a U.S. aircraft carrier since November 2017.

The Ronald Reagan then joined the USS Theodore Roosevelt and Nimitz in a rare three-carrier exercise with South Korean naval vessels during North Korea's last provocative run in nuclear and intercontinental ballistic missile tests.

The latest drill came weeks after President Joe Biden and South Korean President Yoon Suk Yeol in a summit in Seoul vowed to upgrade defense exercises and discuss ways to for Washington to protect its ally in the face of growing North Korean threats.

The U.S. Navy's 7th Fleet said that the exercise was aimed at strengthening the interoperability of the two navies.

On Friday, Biden's special envoy for North Korea, Sung Kim, said Washington is "preparing for all contingencies" in close coordination with South Korea and Japan as it monitors North Korean arrangements for a possible nuclear test that could be imminent.

Kim, who was in Seoul for a trilateral meeting with his South Korean and Japanese counterparts to discuss the nuclear standoff with North Korea, said Washington assesses that the North is pressing ahead with preparations at its nuclear testing ground in the northeastern town of Punggye-ri.

The North's next test would be its seventh since 2006 and the first since September 2017, when it claimed to have detonated a thermonuclear bomb to fit on its intercontinental ballistic missiles.

Nuclear negotiations between Washington and Pyongyang have stalled since 2019 over disagreements in exchanging the release of crippling U.S.-led sanctions for the North's disarmament steps.

North Korea has already conducted missile tests 17 times this year, including its first ICBM demonstrations in nearly five years.