The quake occurred on the headland towards the Aleutian archipelago, right on the coast, which increases the risk of tsunamis.
"Based on preliminary earthquake parameters, widespread dangerous tsunamis are possible," it says.
According to witnesses on social media, the tsunami alert is ringing in Alaska's coastal areas.
The warnings apply primarily to the coasts of Alaska and northwestern Canada, but the United States is also sending messages to other Pacific countries about heightened vigilance.
And any waves are feared to reach as far away as Samoa, at the same latitude as northern Australia.
Japanese authorities say they are investigating whether the country's coasts are in danger.