The three countries have yet to confirm the movements at the base in Jeongju, 100 kilometers (some 60 miles) north of Pyongyang, are for test-firing missiles or for simple training, the largest-circulation South Korean daily Chosun Ilbo said, quoting an unnamed official.
"Since two or three days ago, there have been activities -- such as the relocation of mobile launchers in an apparent bid to test-fire Scud or Rodong missiles," the official said.
"We are following the movements to see if this is just part of a routine training or intended to launch a missile."
Seoul's defense ministry officials refused to confirm the report.
North Korea has short-range Scud missiles targeting South Korea and intermediate-range Rodong missiles with a 1,300 kilometer range which cover most areas of Japan.
Pyongyang stunned the world in 1998 by test-launching over Japan a Taepodong-1 missile with a range of up to 2,000 kilometers.