North Korea said Saturday it had successfully tested an engine designed for an inter-continental ballistic missile (ICBM) that would "guarantee" an eventual nuclear strike on the US mainland.

It was the latest in a series of claims by Pyongyang of significant breakthroughs in both its nuclear weapons and ballistic missile programmes.

Outside experts have treated a number of the claims with scepticism, suggesting the North Korean leadership is attempting to talk up its achievements ahead of a showcase ruling party congress next month.

According to the North's official KCNA news agency, the ground engine test was ordered and personally monitored by leader Kim Jong-Un.

As soon as Kim flagged off the test, "the engine spewed out huge flames with deafening boom", KCNA said.

"The great success... provided a firm guarantee for mounting another form of nuclear attack upon the US imperialists and other hostile forces," Kim was quoted as saying.

Now North Korea "can tip new type inter-continental ballistic rockets with more powerful nuclear warheads and keep any cesspool of evils in the earth including the US mainland within our striking range", he added.

The North's top newspaper Rodong Sinmun featured photos of Kim overseeing the test on its front and second pages Saturday, and said the trial was reportedly conducted at the Sohae Space Centre.

One picture showed Kim looking down from an observation tower as flames were seen over the horizon. Another showed flames streaming down from an engine propped up vertically.

Military tensions on the divided Korean peninsula have been rising since the North conducted its fourth nuclear test in January, and a long-range rocket a month later that was seen as a disguised ballistic missile test.

The UN Security Council responded with its toughest sanctions to date over the North's nuclear programme, and Pyongyang accused Seoul and Washington of spearheading the sanctions drive in New York.

In recent weeks, state media has carried repeated threats of pre-emptive nuclear strikes against both the South and the US mainland.

- Weapons claims -

The threats have been accompanied by claims of success in miniaturising a nuclear warhead to fit on a missile, developing a warhead that can withstand atmospheric re-entry, and building a solid-fuel missile engine.

North Korea has never tested an ICBM, although it has displayed such a missile, known as the KN-08, during recent mass military parades in Pyongyang.

While the North has clearly made progress in developing the KN-08, most experts still believe it is years from obtaining a credible ICBM strike capability.

Kim described the engine test as an "eye-catching event" that demonstrated the North's national defence capability to the world.

He also noted that it represented "another great victory" to be presented at the upcoming Workers' Party Congress, which is believed to be scheduled for May 7.

It is the first congress of its kind for 36 years and seen as a showcase for the leadership to hype its achievements and to cement national unity and loyalty around Kim Jong-Un.

"Kim was showing off his military build-up achievements to consolidate loyalty ahead of the Party Congress as economic difficulties grow following UN sanctions," Professor Yang Moo-Jin of the University of North Korean Studies told AFP.

Some analysts have suggested the North could even conduct a fifth nuclear test before the congress, and South Korean officials say they are fully prepared for such an eventuality.

The North said its January test was of a powerful hydrogen bomb, but experts said the detected yield was too low for a full-fledged thermo-nuclear device.

North Koreans across the country have been mobilised in a "70 day campaign" to prepare for the party gathering, with towns and cities across the country being spruced-up and prettified for the event.