"There is a possibility that North Korea will launch a missile as active movements of vehicles have been detected at a missile research facility in Pyongyang," the National Intelligence Service told a closed-door parliamentary audit, the Yonhap news agency reported.
In July Pyongyang launched two ICBMs apparently capable of reaching the US mainland, described by leader Kim Jong-Un as a gift to "American bastards", and followed up with two missiles that passed over Japan and its sixth nuclear test, sending tensions rocketing.
Trump and Kim have engaged in a heated war of words in recent months, trading threats and personal insults and heightening global alarm.
The US president will arrive in Seoul on Tuesday as part of his Asia tour which also includes Japan, China, Vietnam and the Philippines, with all eyes on his message to Kim and the North.
Amid a flurry of diplomatic visitors to the region, NATO chief Jens Stoltenberg said Thursday that Pyongyang's weapons ambitions have become a "global threat", with the US mainland and Europe coming within reach of its missiles.
But the 29-nation defence alliance was "always ready to respond and to counter any attack from any direction", he added. "That's the way NATO has handled ballistic threats for decades."
It was important not to dramatise the tensions and create a "more challenging situation", he said.
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