North Korea appears to be readying some kind of a rocket launch, two US defense officials said Thursday, amid concerns Pyongyang is preparing to test a ballistic missile in violation of UN Security Council rules.

The officials' comments came after Japanese media reported that satellite images showed North Korea seemed to be setting up a long-range ballistic missile launch from the Dongchang-ri site in western North Korea.

"The indications are that they are preparing for some kind of launch," one US official told AFP, speaking on condition of anonymity.

The official did not say where in North Korea the preparations were taking place, but said people on the ground appeared to be readying "a regular space launch."

"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.

He added, however, that there was nothing to indicate the launch was "ballistic-missile related."

But a second US official, who said the launch was coming "soon," cautioned that North Korea typically uses a space launch as a pretext for developing ballistic-missile capabilities.

"Our concern is that when they do a space launch, it happens to be the same components that can be used in an ICBM," or intercontinental ballistic missile, the official added, also requiring anonymity.

The development parallels events in December 2012, when Pyongyang put a satellite into orbit with its Unha-3 carrier.

The international community condemned the launch as a disguised ballistic missile test, resulting in a tightening of UN sanctions, despite Pyongyang's claim it was a scientific mission.

Citing an anonymous government source, Kyodo News in Japan said the satellite imagery had been collected over the past several days.

Increased movements of people and vehicles were seen around the launch site, which has now apparently been covered over, Japanese national broadcaster NHK said, citing a source familiar with Japan-South Korea relations.

The United States regularly monitors North Korea from space, while Japan began satellite monitoring of the country in 2003.

North Korea is banned under UN Security Council resolutions from carrying out any launch using ballistic missile technology, although repeated small-range missile tests have gone unpunished.

- Fourth nuclear test -

The latest activity comes amid a flurry of diplomatic activity over possible further sanctions against Pyongyang for conducting its fourth nuclear test earlier this month.

Pyongyang said the blast was a miniaturized hydrogen bomb, but experts have largely dismissed the claim.

Washington is pushing for a strong United Nations response, including enhanced sanctions.

But China, North Korea's chief diplomatic protector and economic benefactor, is reluctant, even if Beijing's patience has worn thin in recent years as its neighbor has pursued its nuclear weapons ambitions.

US Secretary of State John Kerry met with his Chinese counterpart Wang Yi in Beijing on Wednesday and said they had agreed to mount an "accelerated effort" to resolve their differences over a new resolution.

Kerry, who said nuclear-armed North Korea poses an "overt threat, a declared threat to the world," acknowledged that the two diplomats had not agreed on the "parameters of exactly what (a resolution) would do or say."

South Korean defense ministry spokesman Kim Min-Seok declined to confirm or deny the Japanese media reports, but said the South's military was monitoring the site for any signs of a long-range missile launch.

"In the past, North Korea always fired a long-range missile ahead of a nuclear test. But since it didn't this time, we are concerned that it could launch one" afterward, Kim said.

Kim also stressed that Pyongyang in the past had notified China and the US before carrying out nuclear tests, though this time it did not.

"We believe that North Korea could launch grave provocations by surprise -- without pre-warning -- from now on."

The North said that it carried out a submarine-launched ballistic missile test in December.

Pyongyang hailed that test as a great success and released a video that researchers at the California-based James Martin Center for Nonproliferation Studies concluded had been heavily edited to cover up a "catastrophic" failure.

The North claims it has developed long-range missiles capable of hitting the United States, but many experts say Pyongyang is still years away from obtaining a credible intercontinental ballistic missile capability.