North Korea has begun fuelling a long-range rocket and could launch it by the weekend, according to news reports.
In a report broadcast by CNN, the network quoted US military officials saying the fuelling signals North Korea is in the final preparation stages for the launch.
The United States and others are threatening punishment for a move they say violates UN 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 US territory.
Officials in Seoul could not confirm the report but a presidential Blue House official who asked not to be named said: "It was well-predicted they might start fuelling at this point."
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.
Pyongyang has threatened to attack any US war planes "meddling" with the planned launch. The United States, which has missile interceptor warships in the region, has said it has no intention to shoot down the rocket.
The launch will be the first big challenge for US President Barack Obama in dealing with the prickly North, whose efforts to build a nuclear arsenal have long plagued ties with Washington.
In London Wednesday, a US 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 UN 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.
North Korea has said it is putting a satellite into orbit and has the right to do so 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 UN 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.
Reuters