The remnants of Hurricane Isaac brought heavy rainfall and the threat of flash flooding to the Mississippi Valley on Friday as Gulf Coast residents cleaned up and energy facilities geared back into operation - in one case with government help.

Major offshore oil drillers were returning staff to their platforms in the Gulf of Mexico, although operations were expected to take several days to ramp up.

The first hurricane to hit the US this year will be remembered for striking New Orleans on the anniversary of 2005's deadly Hurricane Katrina - and providing a first, successful test of the city's new $14.5 billion flood controls imposed after Katrina.

"We are now fully in the cleanup and recovery phase of this storm," said New Orleans Mayor Mitch Landrieu.

Isaac left some homes in the state under 12 feet (3.6 meters) of water. More than 500,000 homes and businesses across Louisiana, Mississippi and Arkansas were still without electricity Friday morning.

At least four deaths were attributed to Isaac in the United States and at least 23 people in Haiti and the Dominican Republic earlier.

The storm caused anywhere from $700 million to $2 billion in insured onshore losses, disaster modeler AIR Worldwide said late Thursday.

That would leave Isaac, which came onshore as a Category 1 hurricane, well outside the 10 most costly U.S. hurricanes.

The National Hurricane Center said Isaac, downgraded to a tropical depression, still was likely to trigger tornadoes in the central U.S. Midwest states - among the final acts of a storm that often confounded forecasters and punched above its weight in terms of damage.

Isaac's rain was a godsend for Midwest farmers suffering from the worst drought in more than 50 years. Even if too late for many of this season's crops, the rain will replenish soil moisture in time for winter wheat planting and boost critically low river levels.

Isaac caused widespread flooding and property damage in the U.S. Gulf Coast region, mostly because of its heavy and persistent rainfall. The system lingered near New Orleans for the better part of two days, sometimes moving as slowly as 5 miles per hour (8 km per hour).

Through it all, New Orleans sustained mostly cosmetic damage such as downed trees and street lights.

Life was beginning to returning to normal in the city on Friday, although most of it was still without power after what utility Entergy Corp described as the fourth-largest storm it had ever faced.

National Guard troops had opened three sites around New Orleans to distribute water, ice and military-style prepackaged meals to residents on a warm, steamy day. Gasoline still was hard to find as well.

New Orleans International Airport reopened early on Friday, and the Port of New Orleans also reopened, in time for the arrival of the 2,052-passenger Carnival Elation cruise ship.

Downtown and in the French Quarter, businesses opened, either with generators or without electricity. Most stores had removed boards from windows, and some commuters headed to work. – Reuters