Hurricane Laura has struck the coast of Louisiana, with extreme winds causing flash flooding in the US state, the National Hurricane Center (NHC) says.
The category four hurricane is expected to cause an “unsurvivable” storm surge as it moves inland with wind speeds of up to 150mph (240km/h).
If it maintains those speeds it would be one of the strongest storms to ever hit the US Gulf Coast.
Half a million residents have been told to leave parts of Texas and Louisiana.
Hurricane Laura made landfall shortly after midnight local time (05:00 GMT) near the district of Cameron, in Louisiana.
The NHC warned any residents remaining in or near the path of the “catastrophic” storm to “take action now to protect your life… in a reinforced interior room away from windows”.
“Get under a table or other piece of sturdy furniture,” the NHC said, adding: “Use mattresses, blankets or pillows to cover your head and body.”
Over 330,000 homes in Louisiana reportedly lost power in the early hours on Thursday, according to the US tracking site Power Outage. In Texas, more than 80,000 homes suffered power cuts.
Television footage showed heavy rain and strong winds lashing the coastal city of Lake Charles in south-west Louisiana, and damage to some buildings has already been reported.
Laura and another storm, Marco, earlier swept across the Caribbean, killing 24 people. Marco struck Louisiana on Monday, bringing strong winds and heavy rain.