North Korea is expected to conduct more test launches of its Iskander-variant ballistic missiles in an effort to replace old Scud missiles with advanced solid-fuel ones, an expert of the state-run defense think tank said Thursday.

Last week, the communist country fired two short-range ballistic missiles, believed to be an upgraded version of its KN-23 missile modeled after Russia's Iskander. The North's state media said they were new tactical guided missiles.

"Russia developed the solid-fuel Iskander missile to replace liquid-type Scud missiles. North Korea is also taking similar steps," Lee Sang-min of the Korea Institute for Defense Analyses (KIDA) told reporters.

"North Korea claimed that the missile fired last week has a range of 600 kilometers. To be a substitute for Scud missiles, this weapon needs to have a longer flight range. So the North is likely to continue upgrading this missile and carry out more test launches, I believe," he added.

Seoul's Joint Chiefs of Staff (JCS) said that the projectiles flew around 450 km.

As major military assets targeting South Korea, the North has Scud-B/C/ER ballistic missiles. Scud-B/C types are believed to fly as far as 500 km, and Scud-ER has a range of around 1,000 km, according to defense ministry data.

Solid-fuel weapons are easy to manage and have advantages in surprise launches, which make them more difficult to detect, according to experts.

The researcher also said that steam detected at the plutonium reprocessing facility at North Korea's Yongbyon nuclear complex does not necessarily mean the country is preparing to extract plutonium for nuclear weapons.

According to Beyond Parallel, a project of the Center for Strategic and International Studies think tank, satellite imagery taken earlier this week showed a plume of steam or smoke emanating from a small support building in the center of the reprocessing plant, known as the Radiochemical Laboratory.

"Smoke from the plant could be part of its regular facility management activities. I don't think we need to react to the matter sensitively," Lee said.

"As for its 5-megawatt reactor, the volume of plutonium from the old reactor would be very small," he said, referring to the North's main source of weapons-grade plutonium. (Yonhap)