North Korea could test-launch several missiles imminently and simultaneously, South Korean military were quoted as saying Thursday.
Several mobile launchers had been deployed on the country's east coast, unnamed military personnel in Seoul were quoted as saying by radio broadcaster KBS.
As many as five mobile launchers have been spotted in the North's eastern province of South Hamkyung, Seoul-based Yonhap News Agency reported Wednesday, citing military satellite imagery.
At least one was loaded with a intermediate-range Musudan rocket, with an untested but estimated range of 3,000-4,000 kilometres, reaching anywhere on the peninsula, Japan and the US military base on the Pacific island of Guam.
"There are clear signs that the North could simultaneously fire off Musudan, Scud and Nodong missiles," Yonhap on Wednesday quoted a government source as saying on condition of anonymity.
Scuds can reach 300-500 kilometres, and Nodongs have a range of 1,300 to 1,500 kilometres.
Tensions have risen since North Korea tested its third nuclear explosion on February 12, triggering renewed sanctions against it.
Pyongyang escalated its rhetoric in response to the sanctions and to military drills by South Korea and its ally the United States in March.
It declared the drills a rehearsal for an invasion and itself in a state of war, as well as making repeated threats of military action including a nuclear strike.
US Secretary of Defence Chuck Hagel said Wednesday the isolated regime was "skating very close to a dangerous line," and South Korea raised its alert level to "vital threat" amid signs that Pyongyang is preparing for a missile test.