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

Pyongyang has said it will send a satellite into orbit between 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.

Senior U.S. military officials quoted by broadcaster CNN on Wednesday said the fuelling indicates the rocket could be ready to launch by the weekend. North Korea's rocket plans have clouded discussions of the G20 meeting of global leaders in London.

Japan has sent missile-intercepting ships along the rocket's flight path, which takes it over the Asian economic power, and said it could shoot down any debris such as falling booster stages, that threatens to strike its territory.

North Korea, which has issued numerous threats concerning the launch, used some of its strongest language in its latest rhetorical blast, with a spokesman for the North's military saying it will "deal a retaliatory lightening of fire" against Japan if it tries to shoot down the rocket.

"If Japan recklessly 'intercepts' the DPRK's (North Korea's) satellite for peaceful purposes, the Korean People's Army will mercilessly deal deadly blows not only at the already deployed intercepting means but at major targets," the spokesman was quoted as saying by the North's KCNA news agency.

The North has deployed the newest jet fighters in its ageing air force to a field near the Musudan-ri launch site to prepare for any contingencies, South Korea's biggest daily Chosun Ilbo quoted government sources as saying.

CHALLENGE TO OBAMA

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 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.

They say the launch would violate U.N. Security Council resolutions imposed after the missile's test in 2006 and the North's only nuclear test a few months later.

Any attempt to punish North Korea will infuriate Pyongyang, which has also 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.

SABRE RATTLING

Traders in Seoul, used to the North's sabre rattling, shrugged off the latest development, but analysts said an attempt to shoot down the rocket would increase the chance of conflict in North Asia, which accounts for one-sixth of the global economy.

The launch is a risk for the cash-strapped North. A failure would hurt missile sales, one of its few export businesses, and embarrass North Korean leader Kim Jong-il, 67, whose suspected stroke in August raised questions about his grip on power.

A successful launch, just ahead of the annual meeting of North's parliament next week, would put to rest any questions about Kim's power, analysts said.