North Korea's recent test of a solid-fuel rocket engine suggests that the communist nation may be trying to develop a new two-stage missile similar to its Rodong medium-range ballistic missile, a U.S. expert said Tuesday.

The North claimed last week that 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.

John Schilling, a U.S. aerospace engineer with expertise on North Korean rockets, said in an article contributed to 38 North, a website specializing in North Korean issues, that analysis of the photos shows that the test was successful.

"Based on the evidence of these pictures, North Korea appears to have conducted a successful test of a large solid-fuel motor of 15-20 tonnes thrust and about one minute of burn duration,"

Schilling said, adding that it's about three times more powerful than the biggest solid motor North Korea has previously demonstrated. 

The expert said the new engine, which is believed to be 1.25 meters in diameter and 3 meters long, appears to be too heavy to be used for any existing North Korean missile systems, including its Unha space launch vehicle and the KN-08 intercontinental ballistic missile.

"Since there are clear disadvantages to using this motor in any existing North Korean missile systems, could a new missile be under development? One possibility is a two-stage solid fuel replacement for the Nodong," Schilling said, using another name for the Rodong.

Both Iran and Pakistan, once major customers of the Rodong, have developed two-stage solid-fuel missiles of similar size and performance but much greater robustness and operational flexibility, the expert said. 

Overall, last week's engine test is certainly a step forward in building solid-fuel motors, but it is only likely to result in an improvement of the regional range ballistic missiles already in the North's inventory, rather than moving the country closer to building an advanced ICBM, the expert said. (Yonhap)