Isolated North Korea said on Wednesday it was injecting fuel into a long-range rocket as we speak ahead of a launch condemned by its neighbours and the West as a disguised long-range ballistic missile test.

The launch, set to take place between Thursday and next Monday, has prompted neighbours such as the Philippines to re-route their air traffic just in case. Regional powers also worry it could be the prelude to another nuclear test, a pattern the hermit state set in 2009.

Japan said it would shoot down the rocket if it crossed its airspace.

The launch of the Unha-3 rocket, which North Korea says will merely put a weather satellite into space, breaches U.N. sanctions imposed to prevent Pyongyang from developing a missile that could carry a nuclear warhead.

It coincides with the 100th birthday celebrations of the founder of North Korea, Kim Il-sung, whose grandson, Kim Jong-un, now rules. Kim Il-sung died in 1994.

It will also follow Wednesday's annual Workers' Party Congress which is expected to appoint Kim Jong-un as Secretary General of the Workers' Party of Korea, North Korea's top post held by his late father, Kim Jong-il.

I think the fuel injection will be completed at an appropriate date, Paek Chang-ho, head of the satellite control centre of the Korean Committee of Space Technology, told a group of foreign journalists in the North Korea capital, Pyongyang.

He would not comment on when the fuel injection would be complete. And as for the exact timing of the launch, it will be decided by my superiors, Paek said.

South Korea, which remains technically at war with the North after their 1950-53 conflict ended with a truce rather than a peace treaty, warned Pyongyang it would deepen its isolation if it went ahead with the launch.

Security sources in Seoul, citing satellite images, have said that North Korea, which walked out of six-party disarmament talks three years ago, is also preparing a third nuclear test following the launch, something it did in 2009, and a move bound to trigger further condemnation and isolation.

U.S. Secretary of State Hillary Clinton warned that history pointed to additional provocations from North Korea after the launch, apparently a reference to a nuclear test.

This launch will give credence to the view that North Korean leaders see improved relations with the outside world as a threat to their system, she told cadets at the U.S. Naval Academy in Annapolis, Maryland.

And recent history strongly suggests that additional provocations may follow.

She also called on China to do more to ensure regional stability.

China, impoverished North Korea's only major ally, on Tuesday reiterated its pleas for calm and said it had repeatedly expressed its concern and anxiety about the developments, Foreign Ministry spokesman Liu Weimin told a press briefing in Beijing.

(Reporting by Maxim Duncan, Writing by Nick Macfie, Editing by Jonathan Thatcher)