The wait for North Korea's expected missile test, which has kept South Korean and US forces on heightened alert for the past two weeks, may stretch to July, the South's defence ministry said Monday.

South Korean intelligence says the North has moved a number of ballistic missiles and missile launchers to its east coast in apparent preparations for a launch amid simmering tensions on the Korean peninsula.

The North has a habit of linking high-profile military tests with key dates, and many experts had predicted a test on or around April 15 -- the birth of North Korea's late founding leader Kim Il-Sung.

Defence Ministry spokesman Kim Min-Seok identified three upcoming dates, including the April 25 founding anniversary of the Korean People's Army and April 30 -- the final day of ongoing South-US military exercises.

One other possibility, Kim said, was the July 27 anniversary of the armistice that ended the 1950-53 Korean War.

"These are three potential dates ... but no one can predict exactly what direction the North will take," he said.

The test is expected to be that of a Musudan missile, which has an estimated range of 2,500 to 4,000 kilometres (2,500 miles), enough to reach South Korea and Japan and potentially US military bases on the Pacific island of Guam.