North Korea is believed to have carried out yet another engine test of its new intercontinental ballistic missile, the KN-08, a U.S. think tank said Wednesday, citing recent satellite imagery.

   The test of the missile's first-stage engine took place in mid-August at the Sohae launch site in the North's northwest, adding to a series of engine tests Pyongyang has carried out since late 2013, said Joel Wit, a security expert who now serves as editor of the 38 North website at Johns Hopkins University.

   "If we see these tests being completed and there's no more, that tells us that the North Koreans may be moving on to possible full-scale tests of the KN-08 in the future because this is a stepping stone to missile tests of that particular weapon," Wit said during a forum on North Korea at the university.

   The missile is believed to have a range of at least 5,500 km, which puts Alaska at risk.

   Wit said he believes that Adm. Samuel Locklear, commander of the U.S. Pacific Command, was referring to the progress the North has made in the development of the KN-08 in a media interview last week.

   Locklear told the Bloomberg Government news service that the North is taking steps to deploy the KN-08. He also expressed concern the missile is harder to keep an eye on as it can be launched from mobile launchers, stressing that building a missile defense system for such weapons is a priority.

   "They are moving forward, as Adm. Locklear said, with the development of a road-mobile ICBM," Wit said.

   Wit said it is difficult to predict when the North would be ready to test-fire the missile.

   Though the missile is not expected to represent a real threat within the next few years, Wit stressed that the North's recent moves show it is "moving forward with it and they are very serious about it."

   North Korea's missile program has long been a key security concern for the region and beyond, along with its nuclear program.

The two programs are designed to ultimately develop long-range nuclear missiles that can reach as far as the mainland U.S.

   Pyongyang has carried out a series of long-range rocket tests.

The latest one, in December 2012, put a satellite into orbit. The country has also carried out three underground nuclear tests, in 2006, 2009 and 2013.

   Frank Jannuzi, a former Senate staffer known for his expertise on North Korea issues, said the latest missile engine test, along with the continued operations at the North's plutonium-producing nuclear reactor, is a result of the failure to hold negotiations with Pyongyang.

   "These activities, the production of fissile material and the progress toward the development of the ICBMs, are activities which could be suspended verifiably if the United States and the other members of the six-party talks were successful in reconvening those dialogue sessions," he said during the forum.

   He called for reopening negotiations with the North before it's too late, stressing that the North's nuclear and missile capabilities are moving forward.

   "I can't tell you when the (KN-08 missile) system will be operational, but I can tell you they're closer now than they were six months ago and they'll be closer six months from now than they are today," he said.

   The six-party talks, which bring together the two Koreas, China, Japan, the U.S. and Russia, have been stalled since late 2008. The North calls for unconditional resumption of the negotiations, but the U.S. and the South demand the North first take concrete steps demonstrating its denuclearization commitment. (Yonhap)