North Korea has issued formal notice of a satellite launch as early as Wednesday, Japan said, with Pyongyang's third attempt to put a military eye in the sky coming after it likely received technical help from Russia in exchange for arms transfers.
North Korea's previous efforts to put a spy satellite into orbit in May and August both failed, and Seoul, Tokyo and Washington have repeatedly warned Pyongyang not to proceed with another launch, which would violate successive rounds of UN resolutions.
But the North has vowed to go ahead anyway, and Seoul's spy agency this month warned the fresh attempt is likely to be more successful as Pyongyang appears to have received technical advice from Russia, in return for sending at least 10 shipments of weapons for Moscow's war in Ukraine.
Japan's coast guard on Tuesday posted a notification on its website of a launch window between November 22 and December 1, and Seoul's Ministry of Oceans and Fisheries quickly issued a navigation warning for ships.
Japanese Prime Minister Fumio Kishida told reporters he would "demand cancellation of the launch... and to make utmost efforts in preparing for unpredictable situations".
He added that any use of ballistic missile technology by Pyongyang would represent a breach of UN resolutions and that Japan was coordinating its response with South Korea and the United States, its partners in a trilateral defence arrangement.
Seoul has warned for weeks that Pyongyang was in the "final stages" of preparation for another spy satellite launch, with defence minister Shin Won-sik saying Sunday that the lift-off could take place as early as this week.
Kang Ho-pil, chief director of operations at the South Korean Joint Chiefs of Staff said Monday that Seoul's military would take "necessary measures to guarantee the lives and safety of the people" if the launch went ahead.
Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP that Seoul was likely to take strong "countermeasures".
President Yoon Suk Yeol "is highly likely to suspend the validity of the September 19 military agreement," he said, referring to a key deal aimed at de-escalating tensions on the peninsula.
The South test-firing its own mid- to long-range solid-fuel ballistic missiles in response could also not be ruled out, he added.
After North Korea's first two attempts to put a spy satellite into orbit both failed, Pyongyang promised to carry out a third in October, which never materialised.
The country is barred by successive rounds of UN resolutions from tests using ballistic technology, and analysts say there is significant technological overlap between space launch capabilities and the development of ballistic missiles.
Russian President Vladimir Putin suggested in September after meeting with Kim Jong Un that his nation could help Pyongyang build satellites.
Seoul and Washington have both subsequently claimed Pyongyang has been shipping weapons to Russia for use in Ukraine, with US Secretary of State Antony Blinken warning this month that military ties between North Korea and Russia were "growing and dangerous".
Successfully putting a spy satellite into orbit would improve North Korea's intelligence-gathering capabilities, particularly over South Korea, and provide crucial data in any military conflict, experts say.
In a commentary carried by the North's official Korean Central News Agency Tuesday, Ri Song Jin, a researcher of the National Aerospace Technology Administration, slammed South Korea's own spy satellite plans, saying they were "extremely dangerous military provocations".
Seoul plans to launch its first spy satellite via a SpaceX rocket later this month, South Korean officials have said.
This shows that the North needs "practical and effective capabilities of space-based reconnaissance and surveillance" as a key way of "exercising the war deterrent more clearly and promoting the strategic security balance in the region," KCNA said.
North Korea has conducted a record number of weapons tests this year.
Seoul, Washington and Tokyo have ramped up their defence cooperation in response, and on Tuesday a US nuclear-powered aircraft carrier, the USS Carl Vinson, arrived at South Korea's Busan Naval Base.
Seoul's military said the visit was to enhance the allies' "posture in response to North Korea's nuclear and missile threats".
"Through close cooperation between the navies of the two countries, we will be equipped with the ability and posture to win even if we fight right now," Seoul's navy said in a statement.