North Korea has been making preparations to launch mid-range ballistic missiles, possibly to mark the birthday of the country's late founding president on Friday, South Korean media reported.
The North deployed one or two Musudan ballistic missiles around the eastern port of Wonsan about three weeks ago, Yonhap news agency said Thursday, citing an unnamed Seoul official.
"There is an ample possibility that the North would launch them around Kim Il-Sung's birthday," Yonhap quoted the official as saying.
The North lavishly celebrates Kim's April 15 birthday, often with massive military parades featuring its most impressive-looking weapons or with missile launches.
The nuclear-armed state has staged several short- and mid-range missile launches but has yet to test the Musudan missile, known to have a range of up to 4,000 kilometres (2,485 miles).
Seoul's defence ministry spokesman said there was a "possibility" the North would carry out such a missile test around Friday's anniversary, but declined to elaborate.
On Tuesday, CNN reported US intelligence satellites had spotted signs North Korea may be preparing for an unprecedented launch of a longer-range mobile ballistic missile.
Seoul military declined to confirm the report.
A US defence official, speaking on condition of anonymity, told AFP that North Korea had been moving several "vehicles" in recent days, including mobile missile systems called "transporter erector launchers."
But he said it was unclear if Pyongyang was actually readying a launch, or if it was just posturing.
"We are always concerned about this type of activity and we will continue to monitor it, but we are confident in our missile defense capability," the official said.
"Nobody actually knows if -- or when -- they are going to launch."
He added that the United States does not believe North Korea has Intercontinental Ballistic Missile capability, and any potential launch would likely be to test an intermediate-range weapon.
Tension has been running high on the divided peninsula since the North conducted its fourth nuclear test in January and a rocket launch a month later that was widely seen as a disguised ballistic missile test.
The UN Security Council responded with its toughest sanctions to date, angering the North, which has since made repeated threats of attacks targeting Seoul and Washington.
They have been accompanied by claims of success in miniaturising a nuclear warhead to fit on a missile and developing a warhead that can withstand atmospheric re-entry.
Outside experts treat the recent claims with scepticism, suggesting leader Kim Jong-Un is seeking to talk up his achievements ahead of a key convention of the ruling Workers' Party in May.