SEOUL (Reuters) - North Korean leader Kim Jong Un supervised a successful test of a new engine for an intercontinental ballistic missile (ICBM), state media said on Saturday, in their latest report of advances in an arms program that has attracted U.N. sanctions.

South Korea and the United States have shown scepticism over the North’s statements about rapid progress in its nuclear and missile programs ahead of a ruling party congress in May, where analysts expect it to declare itself a major nuclear weapons state.

Tension has remained high on the Korean peninsula after the North’s nuclear test and a long-range rocket launch earlier in the year and South Korean and U.S. troops conducted large-scale joint drills amid harsh rhetoric from both rival Koreas.

The engine was ignited at Kim’s command and released a fiery blast, and the test showed the indigenously designed rocket fulfilled all required conditions, the North’s official KCNA news agency said.

“Dear Comrade Kim Jong Un said now we can mount an ever more powerful nuclear warhead on a new intercontinental ballistic rocket and put the den of evil in the United States, and all over the world, within our strike range,” the agency said.

The test was conducted at the North’s missile station near its west coast, where, in February, the country launched a long-range rocket that put an object into space orbit, KCNA said.

South Korea’s defense officials did not immediately provide comment on the authenticity of Saturday’s report.

The North said in March it had miniaturized a nuclear warhead to be mounted on ballistic missiles and conducted a simulated re-entry test of a ballistic missile, which could indicate advances in its ICBM program, if true.

But South Korean officials questioned those assertions and said the North was several years away from developing an ICBM. The United States said there was no proof of the North’s statements and urged Pyongyang to halt actions that fuel tension.

The North conducted its fourth nuclear test in January and the rocket launch in February, in defiance of international warnings and past U.N. sanctions, triggering a new Security Council resolution that imposed more punishment.

Despite its assertions, the North has yet to conduct a flight-test of a long-range missile or an ICBM and show mastery of the technology needed to bring a missile back into the atmosphere and hit a target with precision.

The North said its January nuclear test was a successful hydrogen bomb test, but many experts and officials in the South and the United States said the blast was too small to have been from a successful test of such a weapon.