Isaac is close to the Florida Keys and Louisiana, Mississippi and Alabama have followed Florida in declaring states of emergency.
Hundreds of flights have been cancelled and oil and gas production is affected by a storm that has already killed at least six people in Haiti.
The Republican Party has delayed by a day the start of its convention.
Alabama Governor Robert Bentley has cancelled his visit to the Republican national convention in the Florida city of Tampa and Louisiana Governor Bobby Jindal may do the same, as they prepare emergency responses in their states.

Oil output cut
 
The National Hurricane Center (NHC) said that at 00:00 GMT on Monday, Isaac had wind speeds of 65 mph (105km/h), and could reach hurricane force in the next 24 hours.





Fed by the warm waters of the Gulf of Mexico , Isaac is expected to strengthen to a Category 2 hurricane with winds of 105mph (170km/h), and make landfall on Tuesday or Wednesday, somewhere between Florida and Louisiana.
There is also a chance it may hit New Orleans

on the seventh anniversary of the devastating Hurricane Katrina.
Isaac is also a large storm - it could bring significant damage to areas within 200 miles of its centre.
The Florida Keys are already experiencing rain and strong winds.
Residents have boarded up windows and put down sandbags.
A steady stream of cars has carried people north along the sole road linking the Keys to the Florida mainland.
Hundreds of flights have been cancelled in and out of Miami, Fort Lauderdale and other southern Florida airports.

Power cuts have affected communities from Key West to Fort Lauderdale, hitting more than 6,000 customers.
The US Bureau of Safety and Environmental Enforcement says about 24% of the Gulf of Mexico's oil production and 8% of its gas output have been shut off as a precaution.
Hurricane watches have been put in place along the coastlines of western Florida, Alabama, Mississippi and south-eastern Louisiana.
Florida Governor Rick Scott declared a state of emergency there to make sure emergency services would be ready when the storm hit.
The Republican party has postponed until Tuesday the opening of the convention that will formally nominate Mitt Romney as its presidential candidate
The BBC's Jonny Dymond, in Tampa, says the eye of the storm is forecast to hit the US coastline further north, but given the size of the storm, Tampa is expecting a soaking and powerful winds.
The storm has already caused havoc in the Caribbean, bringing death and considerable damage to Haiti, and floods and downed power lines to Cuba.
Three people were missing in the Dominican Republic, officials said, including the mayor of a town near Santo Domingo.
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