North Korea has moved two more short-range missile launchers to its east coast, according to a South Korean news agency.
An unidentified military source said it indicates the possibility that Pyongyang is planning a missile test.
Satellite imagery suggests the mobile missile launchers are for short-range Scud missiles.
South Korea and its allies have been expecting a provocative act from Pyongyang after weeks of war talk on the Korean peninsula.
Some analysts suggest that North Korea may well choose April 25 for a show of defiance to coincide with the anniversary of the formation of its army.