The North Korean authorities later announced a successful test of what they described as a ‘miniaturized hydrogen bomb.”
The RC 135V returned to base six hours later, Kyodo reported, adding that the spy plane had apparently been keeping an eye on North Korean territory.
Earlier reports said that Pyongyang had not informed other countries about its scheduled nuclear test.
A US reconnaissance plane was on a similar reconnaissance mission along the North Korean border when Pyongyang carried out a nuclear test in 2013.
On Wednesday North Korean television reported a “successful test” of a “miniaturized hydrogen bomb.”
Later in the day South Korea’s Yonhap news agency said, citing a source in the country’s intelligence community, that the explosion of under 6 kilotons of TNT was too slight to have been caused by a hydrogen device.