North Korea is preparing to test new rockets, a report said Thursday, after its leader Kim Jong-Un said the country was in the final stages of developing inter-continental ballistic missiles.

Pyongyang's missile programme and its pursuit of nuclear arms have seen it repeatedly sanctioned by the UN Security Council.

Quoting high-level South Korean officials and South Korean and US military sources, the South's Yonhap news agency said two new missiles had been loaded onto mobile launchers.

They were believed to be equipped with new engines that the North tested last April, it added, when Pyongyang said they would "guarantee" an eventual nuclear strike on the US mainland.

The missiles' existence appeared to have been intentionally leaked by Pyongyang, according to Yonhap, to send a "strategic message" to incoming US president Donald Trump, due to be sworn in on Friday.

CNN and other US news reports, quoting a US defence official, said last week that the Pentagon had deployed a high-tech sea-based X-band radar system to keep watch for a possible North Korean long-range missile launch in the coming months.

A spokesman for the South Korean military's Joint Chiefs of Staff said the Yonhap report had not been verified.

Two US defence officials, speaking on condition of anonymity, told AFP the Pentagon was not seeing signs of any "imminent" launch.

One of the officials said Friday's US presidential inauguration and several political anniversaries in North Korea were fuelling speculation about a possible new launch.

In his closely-watched New Year speech, Kim Jong-Un said North Korea was in the "final stages" of developing an ICBM.

He said the country had significantly bolstered its nuclear deterrent in 2016, pointing to a string of nuclear and missile tests last year.

Analysts are divided over how close Pyongyang is to realising its full nuclear ambitions, especially as it has never successfully test-fired an ICBM.

But all agree it has made enormous strides in that direction since Kim took over as leader from his father Kim Jong-Il, who died in December 2011.

A senior US defence official said last month that the North has developed the capability to pair a nuclear warhead with a missile and launch it.

But it has not mastered the ability to bring the weapon back from space and onto a target, he said.