TOKYO • Speculation that North Korea is preparing a rocket launch following its recent nuclear test mounted yesterday, with American officials and experts backing Japanese reports of suspicious activity at Pyongyang's main satellite complex.

Japan has ordered its military to be ready to destroy any missile fired by North Korea that threatens the country, local media reported yesterday.

A Japanese defence ministry spokesman declined to confirm the report in The Nikkei and Kyodo News that Defence Minister Gen Nakatani has issued the order, saying that "would reveal our strategy".

"But we are taking all possible measures to respond (to a missile launch) by collecting information and coordinating with countries concerned," she added.

With existing United Nations Security Council resolutions banning North Korea from using 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 Jan 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 Agence France-Presse, 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 added.

The US 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.

Citing an anonymous government source, Kyodo News in Japan said satellite imagery showed increased movement at Sohae that could suggest a launch as early as next week.

The main launch site also appears to have been covered over - a precursor to launches in the past.

Observers have noted that North Korea has not yet issued a maritime shipping alert - a standard procedure it has adhered to with previous long-range tests.

The prospect of a rocket launch comes at a time when intense diplomatic efforts are already under way to punish North Korea for its fourth nuclear test.

Washington is pushing for a strong UN response, including enhanced sanctions, but China, North Korea's chief diplomatic protector and economic benefactor, is reluctant.

Pyongyang said it had detonated a miniaturised hydrogen bomb - a claim dismissed by most experts, who say the yield was far too low.

A US official quoted by CNN said the latest assessment, following further seismic analysis, suggested the test, while not of a full-fledged thermonuclear device, may have incorporated H-bomb "components" such as a detonator.

AGENCE FRANCE-PRESSE