North Korea has fired an "unidentified projectile", South Korea's military said late Monday, hours after Pyongyang informed Japan that it was preparing to launch another spy satellite.

"North Korea has fired an unidentified projectile southwards," over the Yellow Sea, South Korea's Joint Chiefs of Staff said, after Seoul, Beijing and Tokyo wrapped up their first trilateral summit since 2019.

Japan also confirmed the launch with the government's alert system briefly issuing an evacuation order to southern Okinawa prefecture.

"Missile launch. Missile launch. It appears a missile(s) was launched from North Korea. Please evacuate inside buildings or underground. Information received at 22:46 (1346 GMT). Target region: Okinawa."

The alert was lifted at 23:03 (1403 GMT), is said, adding "the missile(s) appears not to fly to Japan".

Earlier Monday, after a rare three-way summit of South Korea, Japan and China, the leaders of Seoul and Tokyo urged North Korea to call off the satellite launch.

Nuclear-armed North Korea successfully launched its first reconnaissance satellite last November, drawing international condemnation, with the United States calling it a "brazen violation" of UN sanctions.

South Korea's President Yoon Suk Yeol said another satellite launch -- North Korea's fourth attempt -- would "undermine regional and global peace and stability".

The South Korean military conducted attack formation flight and strike training on Monday to demonstrate "the strong capabilities and will of our military" after North Korea notified Japan of plans to launch a satellite by June 4.

Experts say that spy satellites could improve Pyongyang's intelligence-gathering capabilities, particularly over fierce rival South Korea, and provide crucial data in any military conflict.

Seoul has said the North received technical help from Russia for its November satellite launch, in return for sending Moscow weapons for use in the war in Ukraine.

A group of Russian engineers has entered North Korea to help with the launch preparations, Yonhap reported Sunday, citing a government official.