North Korea has begun fuelling a long-range rocket and could launch it by the weekend, CNN said, with the United States and others promising punishment for a move they say violates U.N. resolutions.

North Korea has said it plans to send a satellite into orbit from April 4-8 but the United States, South Korea and Japan say the launch is a disguised test of the long-range Taepodong-2 missile, which is designed to carry a warhead to U.S. territory.

The fuelling signals North Korea is in the final preparation stages for the launch, CNN quoted U.S. military officials as saying in Washington in a report monitored in Seoul on Thursday. Officials in Seoul could not confirm the report.

The launch will be the first big challenge for U.S. President Barack Obama in dealing with the prickly North, whose efforts to build a nuclear arsenal have long plagued ties with Washington.

In London on Wednesday, a U.S. official, speaking on condition of anonymity on the sidelines of a G20 meeting, said Washington would respond to any North Korean launch by raising the matter in the U.N. Security Council.

The president made clear we are deeply concerned about the prospective missile launch by the North Koreans ... There will be a reaction to it, the official said.

The United States, Japan and South Korea say they see no difference between a satellite and a missile launch because they use the same rocket, the Taepodong-2, which exploded shortly into its only test flight in July 2006.

The three and other global powers said the launch would violate U.N. Security Council resolutions imposed after the earlier exercise in 2006. North Korea has said it is putting a satellite into orbit as part of its peaceful space program.

Any attempt to punish North Korea will infuriate Pyongyang, which has threatened to restart a plant that makes arms-grade plutonium and quit nuclear disarmament talks if the United Nations takes action.

Analysts said they expect China, a veto-wielding member of the U.N. Security Council and the closest thing Pyongyang can claim as a major ally, to block any new sanctions or attempts to tighten the enforcement of existing ones.

Saber RATTLING

Traders in Seoul, used to North Korean saber rattling, mostly shrugged off the latest development, but analysts said any attempt to shoot down the rocket would increase the chances of conflict in North Asia, which accounts for one-sixth of the global economy.

Market participants have learned over time to remain calm to North Korea-related developments, said Lee Kyoung-su, a market analyst at Taurus Investment & Securities.

The launch poses a major risk for the cash-strapped North. A failure would deal a blow to missile sales, one of its few successful export businesses, and embarrass North Korean leader Kim Jong-il, 67, whose suspected stroke in August raised questions about his steely grip on power over Asia's only communist dynasty.

A successful launch, coming just ahead of the annual meeting of North's parliament next week, would put to rest any questions about Kim's power and could help him pave the way for succession, analysts said.

The North Korean people need this. If you have a military first regime (like North Korea's), it has to been seen as doing something. You are going to need these spectacular displays of North Korean defiance of the outside world, said Brian Myers, a professor at South Korea's Dongseo University and a specialist in the North's state ideology.

Several missile-interceptor ships with sophisticated radar from Japan, the United States and South Korea are expected to be in waters along the rocket's flight path over Japan but there are no plans to intercept it unless it threatens their territories.

U.S. spy satellites constantly monitor the launch site in the northeast corner of North Korea. Weather for the area is expected to be partially cloudy from Saturday.

(Additional reporting by Kim Junghyun, Park Jung-youn, Yoo Choonsik and Seo Eun-kyung in Seoul and Caren Bohan in London, Editing by Dean Yates, Jonathan Hopfner and Sanjeev Miglani)