Kim is expected to detail the next phase in North Korea's nuclear weapons programme when he opens a rare congress of the ruling Workers' Party in coming days.

The 600-mm multiple launch rocket system was front and centre as preparations ramped up for the once-in-five-years gathering, widely viewed as North Korea's most important political event.

"When this weapon is used actually, no force would be able to expect God's protection," Kim said, according to the Korean Central News Agency.

"It is really a wonderful and attractive weapon."

Photos released by state media showed dozens of launch vehicles parked in neat rows on the plaza of Pyongyang's House of Culture, which will host the congress.

The weapon was "appropriate for a special attack, that is, for accomplishing a strategic mission", Kim told a ceremony on Wednesday, using a common euphemism for nuclear weapons.

He said the weapons system -- which was presented to the congress as a gift from munitions workers -- would deter unnamed enemies.

"There is no need to further explain about its destructive power and military value," Kim said.

"This is because it can reduce the aimed target to ashes through surprise and simultaneous attack by focusing its destructive energy."

The launch system could fire rockets with an estimated range of 400 kilometres (250 miles), covering all of South Korea, said analyst Hong Min from the Korea Institute for National Unification.

- Political showcase -

"Its primary purpose is to neutralise the combined air power of South Korea and the United States," he told AFP.

"If equipped with tactical nuclear warheads, a single battery firing four to five rounds could devastate an entire airbase."

South Korea's capital Seoul is less than 50 kilometres (30 miles) from the border at its nearest point.

Analysts believe North Korea would use its vast artillery arsenal to launch saturation strikes on the South should fighting break out.

Kim ordered the expansion and modernisation of missile production in the months leading up to the Workers' Party congress.

Pyongyang has also significantly stepped up missile testing.

Analysts say this drive is aimed at improving precision strike capabilities, challenging the United States, and testing weapons before exporting them to Russia.

Pyongyang has said it will hold a landmark congress of the ruling Workers' Party on an unspecified date in late February.

State media has in recent days carried reports of delegates arriving for the gathering, fuelling speculation it could start any day.

The political showcase is expected to lay out North Korea's foreign policy, war planning and nuclear ambitions for the next five years.

Kim said in late January the gathering would detail "next-stage plans" for developing North Korea's nuclear forces.