American satellites detected the transportation of launchers capable of firing intermediate-range ballistic missiles (IRBMs) amid tension over threats by Pyongyang.
The movement is "not believed to be directly related to the threat issued by North Korea last week to strike the waters near Guam," a source told CNN.
But they added the activity was being "watched closely" by the US military.
It comes after James Mattis, the American Defence Secretary, warned a North Korean missile attack "could escalate into war very quickly".
He said the US would "take out" any such launch aimed at its territory, in an apparent reference to missile defence systems around Guam.
North Korean dictator Kim Jong-un said on Tuesday he would hold off on plans to fire four missiles towards the Pacific island and would instead "watch a little more the foolish and stupid conduct of the Yankees," according to a state news agency.
In photos released with the report, Kim was seen pointing at a map showing a flight path for the missiles from North Korea's east coast, flying over Japan towards Guam.
But the island's Homeland Security Adviser George Charfauros said the movement of missile launchers was likely "just a show of force" to mark the country's Liberation Day on Tuesday.