Japan placed its military on alert on Wednesday to shoot down a North Korean rocket if it threatens Japan, while South Korea warned the North will pay a "severe price" if it proceeds with a satellite launch that Seoul considers a missile test.

North Korea should immediately call off the planned launch, which is a violation of UN Security Council resolutions, the South's presidential Blue House said in a statement. Seoul's warning came after the North notified UN agencies on Tuesday of its plan to launch what it called an "earth observation satellite" some time between Feb. 8 and 25.

"North Korea's notice of the plan to launch a long-range missile, coming at a time when there is a discussion for Security Council sanctions on its fourth nuclear test, is a direct challenge to the international community," the Blue House said.

"We strongly warn that the North will pay a severe price ... if it goes ahead with the long-range missile launch plan," it said.

Japanese Prime Minister Shinzo Abe said he would work with the United States and others to "strongly demand" that North Korea refrain from what he described as a planned missile launch.

Japan's Defense Minister Gen Nakatani ordered ballistic missile defence units including Aegis destroyers in the Sea of Japan and Patriot missile batteries onshore to be ready to shoot down any North Korean rocket that threatened Japan.

The rocket is likely to fly over Japan's southern island of Okinawa in the Pacific Ocean at an altitude of several hundred kilometres.

Reports of the planned launch drew fresh U.S. calls for tougher UN sanctions that are already under discussion in response to North Korea's recent nuclear test.

State Department spokesman John Kirby said the United Nations needed to "send the North Koreans a swift, firm message."

Pyongyang has said it has a sovereign right to pursue a space programme by launching rockets, although the United States and other governments worry that such launches are missile tests in disguise.

A spokeswoman for the International Maritime Organization, a UN agency, said the agency had been told by North Korea it planned to launch the 'Kwangmyongsong' satellite.

The International Telecommunication Union, another UN agency, also told Reuters that North Korea had informed it on Tuesday of plans to launch a satellite with a functional duration of four years in a non-geostationary orbit.

North Korea said the launch would be conducted in the morning one day during the announced period, and notified the coordinates for the locations where the rocket boosters and the cover for the payload would drop.

Those locations are expected to be in the Yellow Sea off the Korean peninsula west coast and in the Pacific Ocean to the east of the Philippines, Pyongyang said.

U.S. officials said last week North Korea was believed to be making preparations for a test launch of a long-range rocket, after activity at its test site was observed by satellite.

North Korea last launched a long-range rocket in December 2012, sending an object it described as a communications satellite into orbit.

North Korea also said last month it had successfully tested a hydrogen bomb but this was met with skepticism by U.S. and South Korean officials and nuclear experts. They said the blast was too small for it to have been a full-fledged hydrogen bomb.