The North has stoked international alarm over its banned nuclear missile programme, but it has not launched a missile test since September 15, raising hopes that ramped-up sanctions are having an impact.
However, the South Korean news agency Yonhap cited a government source as saying that a missile-tracing radar was switched on at an unspecified base on Monday, and there had been a flurry of telecoms traffic.
"It's true that active movements have been detected at a North Korean missile base," the source reportedly said. "Signs like those spotted Monday have recently been detected frequently."
"We need to watch a while longer before determining whether the North is preparing a missile launch or gearing up for (its own) winter drill that starts Friday."
A South Korean defence ministry spokesman declined to comment on the report, but similar accounts from Tokyo caused a temporary slump on the stock exchange there.
The Kyodo news agency quoted sources as saying the Japanese government was on alert after detecting radio signals suggesting North Korea might be preparing for a missile launch.
"North Korea might launch a missile within the next few days," one of the sources was quoted as saying.
However, the Japanese sources also said that as satellite images have not shown any missile or moveable launch pad, the signals might only be related to winter training for the North Korean military.
North Korea's leader Kim Jong-Un visited a new catfish farm northeast of Pyongyang, its state media said Tuesday, in the latest of a series of economic outings that have coincided with a lull in weapons testing.
In September the North conducted its sixth and most powerful nuclear test and staged an intermediate-range missile launch over Japan.
But tensions are expected to spike again as the United States and South Korea kick off a large-scale air force drill on Monday in a new show of force against the North.
The five-day exercise, Vigilant Ace, involves 12,000 US personnel and an unspecified number of South Korean service members flying more than 230 aircraft including F-22 Rapter stealth fighters and other cutting-edge weapons at US and South Korean military bases.
Pyongyang routinely condemns such exercises, labelling them preparation for war.
The US last week unveiled fresh sanctions that target North Korean shipping, raising pressure on Pyongyang to abandon its nuclear programme.
Pyongyang condemned the move as a "serious provocation" on Wednesday and warned that sanctions would never succeed.
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