SEOUL (AFP) - North Korean leader Kim Jong Un hailed an "historic" advance in the country's nuclear strike capability, with the successful test of a solid-fuel rocket engine, state media said on Thursday (March 24).

It was the latest in a string of claims by Pyongyang regarding the growing technical sophistication of its nuclear deterrent - accompanied by threats of strikes on the US mainland.

Tensions have been soaring on the divided Korean peninsula since the North carried out its fourth nuclear test on Jan 6, followed a month later by a long-range rocket launch that was widely seen as a disguised ballistic missile test.

Angered by ongoing large-scale South Korea-US military drills, Pyongyang has ramped up the rhetoric in recent weeks, maintaining a daily barrage of threats against both Seoul and Washington.

The solid-fuel test was personally monitored by Kim, who said it would allow for a major upgrade of the North's missile delivery systems that would "strike great horror and terror into the hearts of our enemies", the state-run KCNA news agency said.

Solid-fuel missiles would have distinct advantages - including greater mobility and the ability to launch within minutes - over Pyongyang's current, largely liquid-fuelled inventory.

The North is already understood to use solid fuel for its short-range, road-mobile ballistic missiles, but not for medium-range or untested long-range missiles.

According to KCNA, the solid-fuel test was of a "large output" engine and included separation testing.

"This is an historic and unforgettable day," Kim was quoted as saying.

In recent weeks, North Korea has also claimed to have miniaturised a thermo-nuclear warhead that can fit on a ballistic missile, and the successful test of re-entry technology that would allow the warhead - when carried on a long-range missile - to survive atmospheric re-entry.

Experts say the claims are likely a mix of fact and exaggeration.

However, there is a consensus that North Korea has made strong and steady progress towards its goal of an inter-continental ballistic missile (ICBM) that can accurately deliver a nuclear bomb as far as the continental United States.