South Korea warned Friday of United Nations "countermeasures" after North Korea set dates for a satellite launch seen by Seoul and Washington as a disguised test of a missile which could reach Alaska.

US President Barack Obama also spoke of the "risks" posed by Pyongyang's missile plans, while UN Secretary General Ban Ki-moon said the launch "will threaten peace and security in the region."

The communist state has told the International Maritime Organisation and the International Civil Aviation Organisation (ICAO) it will launch a communications satellite between April 4-8.

"Regardless of whether North Korea fires a missile or launches a satellite, I believe this issue will be raised at the UN Security Council," Seoul's Foreign Minister Yu Myung-Hwan told reporters.

A statement from his ministry predicted "consultation and countermeasures" at the council.

Japan also said it would refer any launch to the UN as a violation of a past resolution.

"We will lodge protests through the United Nations and will continue to urge (North Korea) to abort the project," Prime Minister Taro Aso said.

The nuclear-armed North insists on its right to "peaceful" space research and has said any attempt to shoot down its rocket will be an act of war.

The US and South Korea say its real aim is to test a Taepodong-2 missile, and a launch for any purpose would violate a UN resolution passed after the North's missile and nuclear tests in 2006.

US Secretary of State Hillary Clinton has also raised the prospect of a UN response, but analysts believe permanent Security Council members China and Russia will shy away from further sanctions.

Russia's chief nuclear negotiator Alexei Borodavkin was equivocal when asked in Seoul Thursday if a launch would violate UN resolutions. "Let us wait and see what will be the real technical parameters of this launch," he said.

US State Department spokesman Robert Wood warned against what he called a potentially "provocative act" but would not elaborate on a US response.

The only previous Taepodong-2 test, in July 2006, ended in failure after just 40 seconds of flight. But the Security Council passed a resolution calling for a halt to the programme.

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

The ICAO said Pyongyang had notified it of a launch some time between 0200-0700 GMT on one of the four days in April.

It released a map of two potential danger areas, one off Japan's northwest coast and the other in the Pacific, indicating that a multi-stage rocket would overfly Japan after shedding its first booster.

The main body of the rocket would plunge into the Pacific.

Regional tensions are already high, after Pyongyang in late January cancelled all peace accords with Seoul in protest at conservative President Lee Mying-Bak's tougher cross-border policy.

Since last week it has ordered its military on combat alert and warned South Korean airlines to stay clear of its airspace, in protest at an ongoing US-South Korean military exercise which it sees as a rehearsal for invasion.

The North Monday also shut its border with the South, switching off military phone and fax lines used to approve border crossings.

It reopened the frontier without explanation a day later but closed it again on Friday.

Seoul's unification ministry said the move stranded about 250 people including three foreigners who had intended to return Friday from Kaesong, a Seoul-funded industrial estate built just north of the border.

More than 600 people were blocked from travelling northwards. It was unclear whether crossings would resume Saturday.

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