"South Korean and American intelligence officials spotted the North moving an object that appears to be an intermediate-range missile to its east coast," an unnamed military source told Yonhap, the South Korean news agency, which was quoted by the British Telegraph.
"According to intelligence analysis, it is thought to be a 'Musudan' missile," the source added.
The Musudan missile has reportedly not yet been test fired but the U.S. base in Guam is thought to be within its 2,500-mile strike range.
It was the deployment of this missile that persuaded the United States Wednesday to speed up the installation of an $800 million missile defense system on Guam.
Yonhap said that North Korea may be planning to fire the missile on April 15, the anniversary of Kim Il-sung's birth, a date that is known in North Korea as "The Day of the Sun".
A spokesman for the South Korean Defense ministry declined to comment on the missile. Meanwhile, Japanese media reported that the North had also moved a KN-08 long-range missile to its east coast.
According to a U.S. official who spoke to CNN Thursday, communications intercepts in recent days indicated that Pyongyang could be planning to launch a mobile ballistic missile in the coming days or weeks. "It's unknown whether it would be a test or a strike," the network reported.
The United States is now reportedly "refining its message" toward North Korea. "We are trying to turn the volume down," a Defense Department official told CNN.
At the same time, National Security Council spokeswoman Caitlin Hayden said, the United States urged North Korea's leaders "to heed President Obama's call to choose the path of peace and come into compliance with its international obligations."
"Threats and provocative actions will not bring North Korea the security, international respect, and economic development it seeks," she said.