Hurricane Nate is expected to make landfall on the US Central Gulf Coast in the coming hours as a Category 2 hurricane.
The National Hurricane Centre in Miami said Nate's top sustained winds have recently risen to 90mph, and the core is now about 180 miles south-south-east of the mouth of the Mississippi River.
Nate is accelerating to 26mph and headed north-north-west on a course expected to take it onto the central Gulf Coast on Saturday night.
Forecasters said the hurricane-force winds extend out up to 35 miles, mainly to the east of the eye of the storm.
Nate has already killed at least 21 people in Central America.
In addition to hurricane warnings and tropical storm warnings already in place along a wide stretch of Gulf Coast, a new tropical storm warning has been issued in the Florida Panhandle from east of the Okaloosa-Walton County line to Indian Pass, Florida.
Louisiana and Mississippi officials declared states of emergency, and Louisiana ordered some people to evacuate coastal areas and barrier islands ahead of Nate's expected landfall.
The National Hurricane Centre in Miami projected that Nate would brush by the south-east tip of Louisiana, near the mouth of the Mississippi River, around 7pm on Saturday (1am UK time).
Louisiana Governor John Bel Edwards said he spoke with President Donald Trump on Saturday morning.
"He assured me that LA would have all the assistance we need as we prepare for Nate," the governor posted on Twitter.
A hurricane warning is in effect from Grand Isle, Louisiana, to the Alabama-Florida border and also for metropolitan New Orleans and nearby Lake Pontchartrain.
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Tropical storm warnings extended west of Grand Isle to Morgan City, Louisiana, and around Lake Maurepas and east of the Alabama-Florida border to the Okaloosa-Walton County line in the Florida Panhandle.
In Louisiana, Mr Edwards mobilised 1,300 National Guard troops, some headed to New Orleans to monitor the fragile pumping system there.
The hurricane centre warned that Nate could raise sea levels by 4ft to 7ft from Morgan City, Louisiana, to the Alabama-Florida border. It had already caused deadly flooding in much of Central America.
A White House statement said Mr Trump has authorised the Federal Emergency Management Agency and the Department of Homeland Security to co-ordinate all federal disaster relief efforts.
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