Tokyo put its missile defenses on alert in response to reports of the Musudan deployment.

WSJ: North Korea Fires Missiles Into Sea

U.S. officials believe the North is preparing to test fire a medium-range "Musudan" missile.

NPR: A Look At The North Korea Crisis

The official said the movements are consistent with that of a Musudan missile.

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That would put US bases on Guam within range, although it is not believed that the Musudan has been tested before.

BBC: US warns N Korea missile launch would be 'huge mistake'

Experts and officials say it is highly unlikely North Korea would attempt an unprovoked strike on foreign targets in the region, but North Korea hasn't tested Musudan missiles previously, heightening the risks from any move to proceed.

WSJ: North Korea Reaffirms Commitment to Nuclear Program