North Korea said Thursday that it had successfully conducted a solid-fuel rocket engine test, which if confirmed would be a major step forward in boosting its missile attack capability against South Korea and the United States.

North Korea is known to use liquid propellants for its main ballistic missiles that target South Korea, U.S. bases in the Asia-Pacific region and the American mainland.

Liquid-fuel missiles need to be re-fueled each time before they are launched, so it is difficult to use them on short notice. The use of solid propellants reduces launch preparation time and increases the mobility of the missiles, making it harder to detect signs they had been fired.

The North's state media said that leader Kim Jong Un expressed delight after observing the successful testing of a "large-output solid fuel rocket engine," which made an "earth-shaking" sound as it spit out a large beam of fire.

Kim said that the test will enhance a missile capability that will "mercilessly" strike enemies in an apparent reference to South Korea and the United States.

It is not possible to independently confirm the North's statement or other high-profile announcements from the secretive, tightly controlled communist country.

South Korea's Defense Ministry said later Thursday it needs to analyze the statement.

The North's announcement came two days after it fired five short-range projectiles into the sea in a continuation of weapon launches it has carried out in apparent response to ongoing South Korea-U.S. military drills it sees as a provocation.

Last week, the North launched its first medium-range ballistic missile into the sea since early 2014, ignoring U.N. resolutions against such tests.

The ongoing South Korea-U.S. drills are the biggest ever, and come after North Korea conducted its fifth nuclear test in January and launched a long-range rocket in February.