Nuclear-armed North Korea has announced that it has completed its first military spy satellite and leader Kim Jong Un has approved final preparations for the launch.
This will be Pyonyang's latest step in a series of missile launches and weapons tests in recent months, including a new solid-fuel intercontinental ballistic missile.
Japan expects North Korea to fire the rocket carrying its satellite over the southwestern chain of islands, as it did in 2016.
"We will take devastating measures against ballistic and other missiles confirmed to have landed in our region," Japan's Defense Ministry said in a statement.
South Korea joined Japan in urging the reclusive North to cancel the planned satellite launch, Reuters reported.
"If North Korea continues to move forward, it will pay the price and suffer losses," a spokesman for South Korea's Foreign Ministry said in a statement urging North Korea to withdraw its "illegal" launch plan.
North Korean state media criticized plans by rivals Japan, South Korea and the United States to share real-time data on missile launches, saying the trio was discussing "malicious measures" to tighten military cooperation.