A key indicator of progress to look for in North Korea's potential long-range rocket launch is whether the rocket would re-enter the Earth's atmosphere, a technology essential to the development of intercontinental ballistic missiles, a U.S. expert said Monday.

Concerns have grown that the North could launch a long-range rocket, possibly around next month's ruling party anniversary, after Pyongyang said earlier this month it has the right to the peaceful use of space and would launch satellites.

Such launches are banned under U.N. Security Council resolutions as Pyongyang has long been accused of using long-range rocket launches as a pretext for test-firing intercontinental ballistic missiles. Experts say long-range rockets and ICBMs are basically the same with differences only in payloads.

"There is one area where satellite launches might make a major contribution to North Korea's ICBM program," John Schilling, a U.S.

aerospace engineer, said in a report carried by 38 North, a website run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies.

"An ICBM warhead, unlike a satellite, needs to come down as well as go up. North Korea has never demonstrated the ability to build a reentry vehicle that can survive at even half the speed an ICBM would require," he said.

The North could launch the reentry vehicle into Earth's orbit, "perhaps carrying a scientific payload where a missile warhead would go, and bringing it back down in a controlled fashion" or could put the reentry vehicle under an enlarged payload shroud, then "accidentally" cutting the third-stage burn short, the expert said.

"So we have two warning signs to look for from the North Korean space program. First, using Unha rockets to launch satellites at the same time they are deploying Unha-derived missiles in hardened silos. That might indicate that North Korea is planning to keep an Unha-based ICBM in service long enough to invest in improving its reliability," he said.

"Second, conducting high-speed reentry vehicle tests during satellite launches. The data from those tests would carry over into any long-range missile program. But it's not something they can really keep a secret," he added.

The North is believed to have honed advanced ballistic missile technologies through a series of test launches, including a 2012 launch that succeeded in putting a satellite into orbit. That test is considered the most successful so far.

The test also sparked fears that the North has moved closer to ultimately developing nuclear-tipped missiles that could potentially reach the United States mainland. The country has so far conducted three underground nuclear tests: in 2006, 2009 and 2013. (Yonhap)