Imagery collected over the past several days suggested the launch from the western Dongchang-ri site could come in about a week, Kyodo News said, citing a Japanese government source it did not identify.
Increased movements of people and vehicles were seen around the launch site, which has now apparently been covered over, national broadcaster NHK said, citing a source familiar with Japan-South Korea relations.
The United States regularly monitors North Korea from space while Japan itself began its own satellite monitoring of the country in 2003.
The development parallels events in December 2012, when Pyongyang put a satellite into orbit with its Unha-3 carrier.
More From This Section
Eight days before the 2012 launch, the North also put up a covering over the facility to hide the work from the view of satellites, NHK said.
The international community condemned the 2012 launch as a disguised ballistic missile test, resulting in a tightening of UN sanctions, despite Pyongyang's claim that it was a scientific mission.
Pyongyang said the blast was a miniaturised hydrogen bomb, though experts have largely dismissed the claim.
Washington is pushing for a strong United Nations response, including enhanced sanctions.
But China, North Korea's chief diplomatic protector and economic benefactor, is reluctant, despite ties becoming strained in recent years as Beijing's patience wears thin with its neighbour's ambitions for nuclear weapons.
US Secretary of State John Kerry met with his Chinese counterpart Wang Yi in Beijing on Wednesday and said they had agreed to mount an "accelerated effort" to try to resolve their differences on a new resolution.