WASHINGTON - Speculation mounted Friday that North Korea is preparing a rocket or long-range missile launch to follow its recent nuclear test, with Japan reportedly ordering its military to shoot down any projectile that threatened its territory.

With existing UN Security Council resolutions banning North Korea from the use of ballistic missile technology, any launch would be a further slap in the face of the international community which is struggling to find a united response to the January 6 nuclear test. Following a Japanese report that cited government sources as saying a rocket launch could come as early as next week, two US defence officials confirmed ongoing activity at the North's Sohae satellite complex.

"The indications are that they are preparing for some kind of launch," one US official told AFP, speaking on condition of anonymity. "Could be for a satellite or a space vehicle -- there are a lot of guesses. North Korea does this periodically -- they move things back and forth," the official said.

The United States regularly monitors North Korea from space, while Japan began satellite monitoring of the country in 2003. North Korea successfully put a satellite into orbit with its Unha-3 carrier in December 2012.

Although Pyongyang insisted it was a purely scientific operation, that launch was condemned by the international community as a disguised ballistic missile test and resulted in a tightening of UN sanctions. "Our concern is that when they do a space launch, it happens to be the same components that can be used in an ICBM (inter-continental ballistic missile)," a second US official said.

Since early 2013, North Korea has been upgrading the Sohae launch complex to handle larger, longer-range rockets with heavier payloads, but most experts say Pyongyang is still years away from obtaining a credible ICBM capability that could threaten the US mainland.