Saipan and Tinian in the Northern Mariana islands are expected to be already feeling the onset of damaging typhoon winds.
The US joint typhoon warning centre says Typhoon Nabi has maximum sustained winds of 230 kilometres an hour and gusts up to 277 kilometres an hour.
The maximum significant wave height is 14 metres.
Typhoon shelters in Saipan have opened and residents living near the coastline or in poorly designed structures have been ordered to evacuate.
Small vessels have been ordered to stay in port in Guam, where most schools and all government agencies have shut down.
Guam international airport has been warned of a line of thunderstorms associated with the approaching typhoon.