North Korea has announced it will launch a satellite early next month, the International Maritime Organisation said Thursday, as Washington and Seoul told Pyongyang to scrap what they see as a disguised missile test and the UN expressed concern.

Pyongyang has advised the IMO of the launch at some time between April 4-8, IMO spokesman Lee Adamson told AFP from London.

North Korea also informed the United Nations' International Civil Aviation Organization, a UN spokesman said Thursday.

UN chief Ban Ki-moon expressed concern about the plan.

"I'm concerned about the DPRK's recent move to launch a satellite or long-range missiles," Ban told a press conference on Thursday. "This will threaten peace and security in the region."

In 2006, the UN body imposed sanctions on North Korea for a similar missile test that Pyongyang had insisted was a satellite launch.

The IMO's Adamson said the letter from the North Korean maritime administration gave coordinates of the possible danger areas to shipping.

These were being confirmed with Pyongyang's embassy in London before the IMO sent an advisory to member governments, probably later Thursday.

A South Korean maritime ministry official, citing information from the IMO, said the North had mentioned an area in the Pacific Ocean and another in the Sea of Japan (East Sea).

The rocket's booster will drop into the sea between Japan and the Korean peninsula and its main body will plunge into the Pacific, a foreign ministry official told reporters separately.

Earlier Thursday the North said it had provided "necessary information for the safe navigation of planes and ships" as part of preparations for launching "an experimental communications satellite."

South Korean and US officials believe the real intention is to test a Taepodong-2 missile that could theoretically reach Alaska.

The nuclear-armed North has asserted its right to peaceful space research and says any attempt to shoot down its rocket will be seen as an act of war.

There have been reports for weeks that the North is preparing to test-fire the Taepodong-2 from a base at Musudan-ri on the northeast coast.

The first such test of the missile in July 2006 ended in failure after just 40 seconds. Following the firing, the UN Security Council passed a resolution calling for a halt to the programme.

Three months later, Pyongyang staged an atomic weapons test. It is unclear whether it has the capability to manufacture a nuclear warhead.

Seoul's Yonhap news agency said launch preparations could be completed within two weeks.

The North's newly-elected parliament is expected to meet in early April to re-elect leader Kim Jong-Il as chairman of the National Defence Commission.

Some analysts say this would be seen as an appropriate launch time.

Tensions are already high after Pyongyang ordered its military on combat alert and warned South Korean airlines to stay clear of its airspace. It was protesting at an ongoing US-South Korean military exercise, which it sees as a rehearsal for invasion.

The US intelligence chief Dennis Blair said Tuesday the North does indeed appear to be planning a space launch but that the technology involved is indistinguishable from a missile test.

US Secretary of State Hillary Clinton said Wednesday that the five nations negotiating nuclear disarmament with North Korea -- China, Japan, Russia, South Korea and the US -- were willing to respond "in a variety of ways, including the Security Council."

North Korea also announced it has joined an international treaty and convention on the peaceful use of space. South Korea's foreign ministry confirmed Pyongyang this month sent documents to Russia's foreign ministry and to the United Nations.

"The North's accession to the treaty is aimed at paving the way for claiming that the launch... is to put a satellite in orbit," said spokesman Moon Tae-Young.

"Irrespective of those moves, such a launch would be a breach of UN resolution 1718."

Japan's government said it "would not tolerate" any North Korean act that raises regional tensions. Russia's chief nuclear negotiator Alexei Borodavkin urged caution.

"There should be restraint, there should be an evading of any actions which could undermine security and stability on the peninsula," he told reporters after talks with his Seoul counterpart Wi Sung-Lac.

Asked whether a launch would violate the UN resolution, Borodavkin replied: "Let us wait and see what will be the real technical parameters of this launch."