TOKYO — Less than a month after its purported H-bomb test, North Korea announced Tuesday it is planning a rocket launch as soon as next week. Though speculation of a launch had been growing for about a week, experts say that with underground railways, giant tarps and a movable launch pad structure in place the North is getting a lot better at hiding its preparations.

North Korea's announcement it is preparing a rocket, which it made by informing international organizations of a Feb. 8-25 launch window, comes after what it claimed was its first H-bomb test on Jan. 6 and statements by American and Japanese officials that they were seeing heightened activity at its main rocket facility. The news also came just hours after China's point man on Korean issues arrived in Pyongyang for talks, presumably about the nuclear test, which Beijing has denounced.

North Korea typically informs groups such as the International Maritime Organization of a pending launch so that cautions can be issued for shipping in the area. The IMO's press office in London confirmed it had been informed of the plan on Tuesday.

The announcement ends speculation over whether North Korea was actually preparing a rocket. Though it says the rocket will carry a Kwangmyongsong — or Bright Star — Earth observation satellite, the type of rocket that will be used is not yet clear.

There are indications — including the construction of a new and taller gantry, visible in commercial satellite imagery — that it could be a bigger and better version of the Unha 3 space launch vehicle that lifted off from the Sohae facility in 2012, on the west coast of North Korea.

That would be in line with North Korea's own previous announcements.

The Unha 3 successfully delivered North Korea's first satellite into Earth orbit. A January 2013 report by Rodong Sinmun, the ruling party newspaper, which has since been deleted from its online edition, quoted a scientist as saying there would be a series of launches of observation and communication satellites culminating with Unha 9, which would carry a lunar orbiter. A North Korean space agency official told an AP television crew last year that more satellite launches are planned in the years ahead, but didn't elaborate.

Models of the larger and much more formidable-looking Unha 9 rocket have since been displayed at various events in North Korea, including annual flower shows held in honor of national founder Kim Il Sung and his son, Kim Jong Il. Unha means galaxy.

Although there are important differences, the United States and others have strongly criticized such rocket launches because similar technologies can be used in the development of ICBMs, which North Korea is banned from doing under U.N. restrictions. North Korea says that it has the right to maintain a peaceful space program.

Tightening its punitive squeeze on North Korea, the U.S. Treasury on Jan. 17 announced sanctions on 11 individuals and entities involved in Iran's ballistic missile program, including Iranian officials it said had direct links to North Korea and work being done by the North on "an 80-ton rocket booster."

It said two of the sanctioned Iranians "have been critical to the development of the 80-ton rocket booster, and both traveled to Pyongyang during contract negotiations." Iran has, coincidentally, suggested it might also conduct a rocket launch this month.

Whether the booster would be a new first stage for the Unha rockets or something different is not known.

Making firm predictions has become more difficult because of the increasingly sophisticated concealing measures North Korea has been developing over the past several years.

Though the kind of intelligence available to agencies in the U.S. and its allies is presumably far better than what they are willing to let on publicly, it is clear from unclassified commercial satellite monitoring that a lot has been going on at North Korea's main launching facility since the 2012 Unha 3 launch.

Concealment upgrades include the construction of an underground railway right up to the launch pad that allows rocket stages to be transported stealthily to the site, possibly from Pyongyang. Upon arrival, the stages could be lifted directly from the train by elevator into a structure above for assembly and then moved in another newly built mobile structure by rail to the gantry, which recently has been covered by a huge tarp.

"In effect, you're not going to see any of that happening, whereas before they would be moving stuff around on trucks that you might be able to spot. So that really makes things much more difficult," said Joel Wit, a former State Department official and editor of the respected 38 North website, which focuses on North Korea issues.

Wit said some activity still cannot be concealed, but with the upgrades "it becomes more like reading tea leaves."

North Korea's efforts at concealment appear to be paying off on the nuclear front as well.

The paucity of detailed intelligence due to a deeper blast at the end of a longer tunnel, better sealing of tunnels and vents and improved camouflage at its nuclear site has muddied analysts' ability to assess North Korea's claim that the latest test was the successful detonation of its first hydrogen bomb — which if true would mark a significant advance in the North's nuclear technology.

In the case of rocket preparations, North Korea knows when commercial satellites are over its area because most are synchronized with the sun and pass over the same point of the Earth at about the same time. For North Korea, that's between 10 a.m. and 1 or 2 p.m. so they tend to stop activity during that window, or move trucks under cover or keep activity in buildings to prevent detection.

Such moves would not be as effective against the best government-operated spy satellites, which can provide almost constant surveillance of high-interest sites and can also see through clouds and the cover of darkness.

But Joe Bermudez, an expert on North Korea's military and the chief analytics officer of AllSource Analysis Inc., said the country's effort to improve its "camouflage, concealment and deception" has been quite elaborate.

"The North Koreans are not stupid people," said Bermudez, who has been studying North Korean defense and intelligence affairs for 35 years, and since the 1990s has been using satellite imagery analysis. "Particularly those in the intelligence community and those involved in the programs to produce WMD watch and read the foreign press. As we use commercial satellite imagery to identify activities in North Korea, they are watching and looking at what we are seeing and they are making adjustments."

------

Associated Press writers Matthew Pennington in Washington and Hyung-jin Kim in Seoul, South Korea, contributed to this report.