North Korea could test a rocket booster developed jointly with Iran or the road-mobile KN-08 intercontinental ballistic missile in its next long-range rocket launch, feared to take place in coming weeks, U.S. experts said.

Satellite imagery has shown increased activities at the North's Sohae, also known as Dongchang-ri, rocket launch site, raising concern that the communist nation may be preparing for a long-range missile or rocket launch just a few weeks after its fourth nuclear test.

A new launch would come after the North completed years of construction to expand the facility to make it possible to launch a larger rocket. The North last conducted a rocket launch from there in 2012, successfully putting a satellite into orbit aboard the Unha-3 rocket.

"North Korea might launch another Unha-3 or a similarly sized rocket, but there is a more interesting possibility," said Jeffrey Lewis, a nonproliferation expert and director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies. 

"North Korea has increased the height of the original gantry at Sohae and displayed models of something much larger called the Unha-9 in strange places like flower exhibitions and Moranbong Band concerts," he said in an article at the Arms Control Wonk blog.

Lewis also noted a now Treasury Department-confirmed report that North Korea and Iran have been cooperating on the development of a new 80-ton rocket booster, raising the possibility of the Unha-9 rocket using the booster as its first stage.

While announcing fresh sanctions on Iran last month, the Treasury confirmed that Tehran and Pyongyang have cooperated to develop a 80-ton rocket booster, with Iranian missile technicians visiting the North for the project in the past several years.

The expert also noted a Iranian news agency report that Iran would launch a Simorgh rocket during the festivities to mark Iran's revolution, which should run from today through Feb. 11. He said Iran's Simorgh has some obvious similarities to the North's Unha rocket.

Gordon Chang, an East Asia security expert, also raised the possibility that Iranians could attend the North's upcoming launch as they have founded the development of the new rocket booster. He also voiced concern that the North's new rocket could end up in Iran.

"All signs point to a long-range missile launcher test in the coming days and not only could this one possibly reach the United States, but it will also likely end up in Tehran's hands," he said in an article to the online news outlet Daily Beast.

Gordon also said the North could conduct its first test of the KN-08 intercontinental ballistic missile.

He cited Joseph DeTrani, a former U.S. intelligence official and negotiator with North Korea, as saying that the communist nation is "close to launching a long-range, solid-fuel, mobile missile capable of reaching the United States."

"There is only one missile in Pyongyang's inventory that could possibly fit that description: the fearsome KN-08," Gordon said.

U.S. military officials have expressed serious concern about KN-08, saying it is harder to keep an eye on as it can be launched from mobile platforms.

In April last year, Adm. William Gortney, commander of the U.S. Northern Command, said the North is believed to be capable of miniaturizing nuclear warheads to put on the KN-08 missile and fire it at the U.S. mainland. (Yonhap)