An undated photo provided on March 29, 2026 by the North Korean government, shows what it says a solid-fuel engine test at an undisclosed place in North Korea. Korean Central News Agency/Korea News Service via AP

North Korean leader Kim Jong Un observed a test of an upgraded solid-fuel engine for weapons capable of reaching the U.S. mainland, and called it a significant development boosting his country's strategic military arsenal, state media reported Sunday.

While the test was in line with Kim's stated goal of acquiring more agile, hard-to-detect missiles targeting the U.S. and its allies, some experts speculate North Korea's claim may be an exaggeration. Missiles with built-in solid propellants are easier to move and conceal their launches than liquid-fuel weapons, which, in general, must be fueled before liftoff and cannot last long.

The official Korean Central News Agency reported that Kim watched the ground jet test of the engine using a composite carbon fiber material. It said the engine's maximum thrust is 2,500 kilonewtons, up from about 1,970 kilonewtons reported in a similar solid-fuel engine test in September.

An undated photo provided by the North Korean government shows what it says is the engine test at an undisclosed site. The event was not covered by independent reporters. The photo shows a large jet of flame shooting from the missile.

KCNA reported the test was conducted as part of the country's five-year arms build-up meant to upgrade "strategic strike means," a term referring to nuclear-capable ballistic missiles and other weapons. Kim said the latest engine test had "great significance in putting the country's strategic military muscle on the highest level," according to KCNA. The agency did not say when or where the test occurred.

North Korea's report on the latest test could be "bluffing" as it didn't disclose some key information like the engine's total combustion time, said Lee Choon Geun, an honorary research fellow at South Korea's Science and Technology Policy Institute.

When North Korea reported about the previous engine test in September, it described it as the ninth and final ground test of a solid-fuel engine that it earlier said would be used for intercontinental ballistic missiles. Observers predicted at the time that North Korea would soon test-launch an ICBM loaded with that engine, but it hasn't done so yet.

North Korea's solid-fuel engine program may be facing some delays or the country might have determined to develop a better engine, possibly with Russian assistance, Lee said. Cooperation between the countries has deepened in recent years, with the North sending troops and conventional weapons to support Russia's war against Ukraine.

In recent years, North Korea has test-fired a variety of ICBMs demonstrating the potential range to reach the U.S. mainland, including solid-fuel ones. But some of North Korea's past claims about major weapons tests drew outside skepticism. In 2024, North Korea claimed to have successfully test-launched a multiwarhead missile, but South Korea quickly dismissed it as a deception to cover up a failed launch.

Several long-range missiles, called Hwasong-20s, were put on display during a North Korean military parade celebrating Kim in October 2025. State media described the devices as the country's "most powerful nuclear strategic weapon system."

Some foreign experts say North Korea still faces technological hurdles before it has a functioning ICBM, such as ensuring its warheads survive the harsh conditions of atmospheric reentry. But others dispute that assessment, given the number of years the country has spent on its nuclear and missile programs.

Possession of more powerful and efficient solid-fuel engines would allow North Korea to build smaller ICBMs that can be launched from submarines or land-based mobile launch trucks, Lee said. Other observers say a push to increase the engine power is likely associated with efforts to place multiple warheads on a single missile to increase chances of defeating U.S. defenses.

North Korea has pushed hard to expand its nuclear arsenal since Kim's high-stakes diplomacy with U.S. President Donald Trump collapsed in 2019. In a ruling Workers' Party congress in February, Kim left open the door for dialogue with Trump but urged Washington to drop demands for the North's nuclear disarmament as a precondition for talks.

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