Imagery collected over the past several days suggested the launch from the western Dongchang-ri site could come in about a week, Kyodo said.
While the report did not provide any details on the source of the analysis, Japan’s key security ally the United States regularly monitors North Korea from space while Japan itself in 2003 began its own satellite monitoring of the country.
South Korean defence ministry spokesperson Kim Min-Seok declined to confirm or deny the report, saying the ministry did not comment on intelligence matters.
He added, however, that South Korea’s military was monitoring for any signs of a long-range missile launch.
The report came as the international community discusses further sanctions against Pyongyang for conducting its fourth nuclear test earlier this month.
“The reclusive country’s supposed action could be aimed at flouting the (UN security) council and any plans among its members to tighten sanctions over the latest nuclear detonation,” Kyodo said in its report.
Washington is pushing for a strong United Nations response, including enhanced sanctions, to respond to the North’s latest atomic blast.
But China, North Korea’s chief diplomatic protector and economic benefactor, is reluctant, despite their 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 Wednesday and said they had agreed to mount an “accelerated effort” to try to resolve their differences on a new resolution.
But Kerry, who said nuclear-armed North Korea poses an “overt threat, a declared threat to the world”, acknowledged that the two had not agreed on the “parameters of exactly what it would do or say”.
Pyongyang said the blast earlier this month was a miniaturised hydrogen bomb, though experts have largely dismissed the claim.
The South Korean defence ministry’s Kim added that officials were on guard for any changes in North Korea’s previous behaviour.
“In the past, North Korea always fired a long-range missile ahead of a nuclear test. But since it didn’t this time, we are concerned that it could launch one” afterwards, he said.
Kim also stressed that Pyongyang used to notify China and the US before carrying out nuclear tests, though this time did not.
“We believe that North Korea could launch grave provocations by surprise — without pre-warning — from now on.”
The possible preparation of a missile launch also came after the North said it carried out a submarine-launched ballistic missile test in December.
Pyongyang hailed that test as a great success and released a video that researchers at the California-based James Martin Center for Nonproliferation Studies concluded had been heavily doctored and edited to cover up a “catastrophic” failure.