Japan military on alert over North Korea's planned rocket launch

* Japan puts ballistic missile defence units on alert

* South Korea warns North will pay "severe price" for launch

* North says it plans to launch satellite Feb. 8-25

* South Korea, Japan describe plan as a missile launch (Updates with comment from China, South Korea)

By Jack Kim and Nobuhiro Kubo

North notified U.N. agencies on Tuesday of its plan to launch what it called an "earth observation satellite" some time between Feb. 8 and 25.

North Korea has said it has a sovereign right to pursue a space programme by launching rockets, although the United States and other governments suspect that such launches are in reality tests of its missiles.

"We have defences ready to deal with all threats, but in view of the announcement I have put the Self Defense Force's Aegis destroyers and our PAC-3 units on alert and issued an order to shoot down any ballistic missile threat," Japan's defence minister, Gen Nakatani, told media briefing.

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.

Tension rose in East Asia last month after North Korea's fourth nuclear test, this time of what it said was a hydrogen bomb.

A rocket launch coming so soon after the nuclear test would raise concern that North Korea plans to fit nuclear warheads on its missiles, giving it the capability to launch a strike against South Korea, Japan and possibly targets as far away as the U.S. West Coast.

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

South Korea said the North should immediately call off the launch, which is a violation of U.N. Security Council resolutions, the South's presidential Blue House said in a statement.

"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.

'EXTREMELY CONCERNED'

China, under U.S. pressure to use its influence to rein in the isolated North, said North Korea's right to space exploration was restricted under U.N. resolutions.

China is North Korea's sole main ally though China disapproves of its nuclear programme.

"We are extremely concerned about this," Chinese Foreign Ministry spokesman Lu Kang told a briefing.

"In the present situation, we hope North Korea exercises restraint on the issue of launching satellites, acts cautiously and does not take any escalatory steps that may further raise tensions on the Korean Peninsula."

Reports of the planned launch drew fresh U.S. calls for tougher U.N. sanctions that are already under discussion in response to North Korea's Jan. 6 nuclear test.

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

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

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.

South Korea told commercial airliners to avoid flying in areas of the rocket's possible flight path during the period.

The launch is likely to be from the North's Tongchang-ri station near the Chinese border.

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

(Additional reporting by Ju-min Park in Seoul and Ben Blanchard in Beijing; Editing by Tony Munroe and Michael Perry, Robert Birsel)