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.
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.
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A spokesman for the South Korean military's Joint Chiefs of Staff said the Yonhap report had not been verified.
In his closely-watched New Year speech, Kim Jong-Un said North Korea was in the "final stages" of developing an ICBM.
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.