Story highlights U.S. military hasn't concluded that long-range rocket launch is imminent, official says

Satellite images show trailers carrying the first two stages of a rocket

A launch could take place early next month, a U.S. academic website says

But it notes that North Korea hasn't announced plans, which it did for previous launches

North Korea is another step closer to the unusual and provocative move of launching a long-range rocket in wintertime, according to an analysis of satellite images by a U.S. academic website.

Using commercial satellite imagery, the website 38 North says that trailers carrying the first two stages of one of the North's Unha rockets can be seen near the main missile assembly building at the Sohae Satellite Launch Station on the country's west coast.

The analysis published Thursday by 38 North, which is run by the School of Advanced International Studies at Johns Hopkins University, follows the release of an image this week by the satellite imagery company DigitalGlobe that showed increased activity at the launch station

The developments shown in the images suggest North Korea could carry out a rocket launch as soon as "the latter half of the first week of December, weather permitting," 38 North said.

Such a move would be surprising and unprecedented , since the nuclear-armed North hasn't before tried to launch a long-range rocket in the winter or so soon after a previous attempt.

JUST WATCHED Economic change for North Korea? Replay More Videos ... MUST WATCH Economic change for North Korea? 02:53

JUST WATCHED Look inside would-be assassin's tool kit Replay More Videos ... MUST WATCH Look inside would-be assassin's tool kit 02:50

A rocket launched from the Sohae site in April broke apart shortly after takeoff; the North's decision to go ahead with that launch drew international condemnation.

Pyongyang said the rocket was supposed to put a satellite into orbit, but the launch was seen by many other countries as cover for a ballistic missile test.

The Pentagon and the intelligence community are scouring classified and commercial imagery of the site and have not firmly concluded that North Korea is planning to launch a long-range rocket, a senior U.S. military official said.

While not discounting the possibility of a launch, the U.S. military is leaving open the chance that other motives could be involved, the official said.

"They could be moving things around just to make a point," the official said. "But on the other hand, it's the North Koreans, so who knows?"

The analysis by 38 North noted that before previous launches, Pyongyang has announced "dates and hours for sea or air closure areas for the rocket's first and second stage impact areas," as well as filing documents for satellite frequency.

"Since that has not happened yet, the window would appear to be closing for an early launch," the website says.

It suggested the possible motivation for the activity at the Sohae site was the planned launch of a satellite-bearing rocket this week by its archrival and neighbor, South Korea. Seoul postponed that launch minutes before takeoff after the discovery of an electronic signal problem.

There was no apparent mention of a planned launch by the North's state-run Korean Central News Agency on Friday.

Another big rocket launch by North Korea could further sour its relations with the United States and South Korea. The failed launch in April scuppered a deal for Washington to provide thousands of tons of food aid to the North's malnourished population.

It also drew condemnation from the U.N. Security Council, which repeated demands for Pyongyang not to carry out similar tests in the future. The botched launch followed attempts in 2006 and 2009.

The 38 North analysis was based on DigitalGlobe satellite images released on November 23 and 26. The analysis was carried out by Nick Hansen, who has specialized in image technology during a 43-year career in intelligence for the U.S. military and private sector.