PYONGYANG • North Korea yesterday showed off what it said were long-range ballistic missiles carrying miniaturised nuclear warheads, which state television said could destroy enemies in a "sea of fire".

Rows of the intercontinental ballistic missiles known as KN-08, which some think could fly far enough to reach the continental United States, were paraded through the capital as part of a massive military parade marking the 70th anniversary of its ruling Workers' Party.

"The party's revolutionary armament means we are ready to fight any kind of war waged by the US imperialists," leader Kim Jong Un said in a speech strikingly more forceful than previous public comments.

The young leader was accompanied by senior Chinese Communist Party official Liu Yunshan, with whom he was seen speaking throughout the event and occasionally sharing laughs, and flanked by senior North Korean party and military officials.

A defence analyst in Seoul said the new missiles appeared to have been modified to allow them to be fitted with miniature nuclear warheads - a claim echoed by North Korea's state TV - but others experts said it was impossible to tell.

North Korea has long claimed it has technology capable of launching nuclear bombs at its distant enemies, but experts are sceptical whether the impoverished country has acquired the sophisticated technology needed to produce such weapons.

"With the vengeful desire to turn the citadel of our enemies into a sea of fire, our powerful tactical rockets loaded with diversified and miniaturised nuclear warheads are on the move," the TV commentary said, as rows of missiles were shown on screen.

Mr Lee Il Woo, a defence analyst at the Korea Defence Network, said the new version of the KN-08 - which has an estimated range of up to 12,000km - had a more rounded end than the version unveiled in 2012.

"This means North Korea might have successfully developed technology to minimise nuclear warheads and fit them on top of missiles," he said.

But another analyst, Mr Chae Yeon Seok at the Korea Aerospace Research Institute, cautioned that the missiles might be mock-ups.

"You never know what is inside by just looking at them. It has never been verified that North Korea has developed any nuclear-tipped ICBMs," he said, referring to intercontinental ballistic missiles.

Seoul's Yonhap news agency, quoting military sources, also said it remained unclear whether the warheads on display might be ordinary explosives or nuclear devices.

In a letter delivered by Mr Liu, the most senior Chinese official to visit Pyongyang since Mr Kim came to power following his father's death in 2011, Chinese President Xi Jinping said China attached vital importance to its relationship with North Korea, China's official Xinhua news agency reported.

China is North Korea's chief ally and its main trading partner, although ties have been strained over the North's nuclear programme.

Mr Xi said in the letter that China had "been striving to treat the bilateral relations from a strategic and long-term perspective".

Mr Kim, who is in his early thirties, told the visiting Chinese delegation last Friday that North Korea was also keen to bolster ties, the North's official KCNA news agency said yesterday.

Mr Liu is the fifth-ranked member on China's ruling Communist Party's elite Politburo Standing Committee. He is in charge of the country's ideology and propaganda work.

"Liu standing next to Kim is a sign both China and North Korea feel it is about time they restored their traditional relations," said Mr Koh Yu Hwan, a professor of North Korea studies at Dongguk University in Seoul.

"China showed its respect by sending a senior delegation and North Korea showed its respect by placing him next to its leader."

AGENCE FRANCE-PRESSE, REUTERS, BLOOMBERG