The solid-fuel rocket engine North Korea tested this week appears to be for the upper stage of a satellite launcher or a long-range missile, but the communist nation is believed to be still many years away from developing such missiles, a U.S. expert said Thursday.

North Korea's state media claimed earlier Thursday the country successfully conducted a ground test of a solid-fuel rocket engine, with leader Kim Jong-un saying that the test "helped boost the power of ballistic rockets capable of mercilessly striking hostile forces."

The North also released photos of the test, showing the engine spewing flames.

A solid-fuel missile would pose a greater threat as it would take less preparation time and would be harder to detect before a launch. South Korea said the North appears to be in the early stages of developing solid-fuel rockets. 

Michael Elleman, a missile engineer and senior fellow at the International Institute for Strategic Studies, said that the engine shown in the photos was obviously a solid propellant motor of standard design.

The engine was short, but wide in diameter, which suggests it's not a first-stage motor, he said.

One of the photos showed the control room of the test, which suggested the rocket motor burned for about 57 seconds, which he said is too long for a first-stage rocket.

"The long burning time as indicated by the control room panel and the low-pressure burn would suggest that this might be designed for the upper stage of a multi-stage rocket system," Elleman said during a briefing organized by 38 North, a website specializing in North Korean affairs and run by the U.S.-Korea Institute at Johns Hopkins University.

It is unclear why the North is pursuing solid-fuel missiles when it's been successful with liquid-propellant systems for its Scud, Nodong, Musudan and KN-08 missiles, but Pyongyang may not be "comfortable with the relatively low performance of engines that they have and they want to go to something more advanced," Elleman said.

Despite progress in its missile programs, the North is not believed to be able to make the engines itself, the expert said, adding that the country probably still relies on Soviet engines. 

"If that's true, then they have a limited stockpile," he said.

To grow the stockpile, they would have to design their own engines, he added. That would be a long, difficult process that will also cost a lot of money and require a "very steady disciplined effort," the expert said, adding that none other than major powers have succeeded in that.

"So it may be that they're looking to move in a different direction similar to what Iran was apparently doing five, six, seven years ago," he said. "It's going to present a lot of challenges. I wouldn't imagine seeing even a 300 km-range, solid-propellant missile out of North Korea in the next five years."

What was surprising in the pictures released by the North was the lack of safety measures, he said.

The photos showed leader "Kim Jong-un standing in front of the rocket motor, which presumably was loaded with rocket propellant, solid-rocket propellant, essentially an explosive that burns at a slower rate," the expert said.

"In other words, they were putting Kim Jong-un at risk by having him near such a volatile system. The risks are low, but nonetheless, they are there," he said. "I did notice that in some of the shots, there was no indication of lightning protection."

Meanwhile, the State Department urged the North to refrain from actions raising tensions.

"We have seen a North Korean state media report of a solid fuel rocket engine test," State Department deputy spokesman Mark Toner said. "Our reaction is that North Korea should refrain any actions and any rhetoric that raise tensions in the region and comply with its international obligations and commitments." (Yonhap)