Washington has been pressing the uneasy neighbors to work more closely to better counter rising threats from China and North Korea.

Seoul and Tokyo have strained relations over disputes dating to Japan's 1910-1945 occupation of Korea.

Colonel Dave Butler, a spokesman for U.S. Chairman of the Joint Chiefs of Staff General Mark Milley, told Reuters that the North Korean launch occurred at the conclusion of the meeting, which had been long planned.

He said the North Korean ballistic missile had been launched towards the Sea of Japan.

This year, North Korea has test fired its first ever solid-fuel intercontinental ballistic missile and conducted a failed attempt to launch its first-ever spy satellite on a new launch vehicle. U.N. Security Council resolutions ban North Korea's use of ballistic missile technology, including for satellite launches.

The trilateral meeting was at Camp Smith in Hawaii and it last took place between the three uniformed military chiefs in March 2022.

North Korea's Kim Yo Jong, the powerful sister of leader Kim Jong Un, said on Tuesday a U.S. military spy plane had entered North Korea's Exclusive Economic Zone eight times. read more

She warned that U.S. forces would face a "very critical flight" if they continued "illegal intrusion", repeating an accusation North Korea made on Monday that the U.S. had violated its airspace by conducting surveillance flights.

North Korea warned that such flights may be shot down.

A senior U.S. defense official, speaking on the condition of anonymity, said it appeared that North Korea's threats were largely bluster.

The official said North Korea's ground based missiles did not have the range to shoot down U.S. planes, which were not flying in Pyongyang's Exclusive Economic Zone as defined by international law.

"It's unlikely to manifest as a significant threat," the official said.

Milley is set to travel to Japan and South Korea this week.