Increased activity has been observed at about 10 missile bases in North Korea and this could be seen as preparation for a launch, a Japanese press report said Sunday.
But the presence of missiles themselves and their launch pads have not been confirmed, the major daily Yomiuri Shimbun said, quoting Japanese and US government sources.
"The chances of an actual Rodong missile launch are slim. We can't rule out the possibility that the activity was just a large-scale military drill," the daily quoted one senior official as saying.
On Thursday, South Korean military officials said data collected jointly with US satellites and spy planes suggested North Korea might be preparing to test-fire a missile.
Missile and other units of North Korea's three services have been acting on orders mainly coming from a Rodong missile base in Sinori, north-west of Pyongyang, Yomiuri quoted the sources as saying.
A Rodong missile, with a range of some 1,300 kilometers (810 miles), can hit most areas in Japan.
Near these bases, vehicles carrying liquid missile fuel, other military vehicles, military personnel and missile engineers were seen gathering, the sources said.
The data have been collected by US and Japanese spy satellites and radio monitors and electronic surveillance aircraft, Yomiuri said.