South Korea and the US have held their first joint military exercise in more than four years involving a US aircraft carrier, the South Korean military said on Saturday amid reports that North Korea is preparing for a nuclear test, Reuters reported.

The three-day exercise took place in international waters off the Japanese island of Okinawa through Saturday, including air defense, anti-ship, anti-submarine and maritime interdiction operations, South Korea's Joint Chiefs of Staff (JCNS) said.

The exercise comes amid signs that North Korea is preparing to conduct a nuclear test for the first time since 2017. Seoul officials said Pyongyang had conducted numerous experiments with a detonation device in preparation for a seventh underground explosion.

The 100,000-ton nuclear-powered aircraft carrier USS Ronald Reagan, as well as the guided-missile cruiser USS Antietam, the Aegis-equipped destroyer USS Benfold, and the USNS Big Horn, an oil tanker fleet replenishment ship, joined the exercise.

South Korea also sent the 14,500-ton amphibious assault ship Marado, the 7,600-ton destroyer Sejong the Great, the 4,400-ton destroyer Munmu the Great and others.

The aircraft carrier USS Abraham Lincoln led a U.S. military exercise in the Yellow Sea in March, after North Korea conducted its first full ICBM test since 2017. The US Navy strike group Abraham Lincoln also operated off the Korean Peninsula in April.