Four million people had lost electricity across a wide area including Miami and traffic signals were malfunctioning, causing congestion on roads and forcing some shops to close.

"We have a massive power outage in Miami, but we don't know still the source of the problem," Cathy Webb, spokeswoman for the Miami Dade Police, told AFP.

"Florida Power and Light is working on that," she said, referring to the local utility company.

CNN reported five reactors at the Turkey Point nuclear power plant had gone down and two were now back online.

Police said there was no sign the outage was due to foul play or criminal activity, US media reported.

"The information we have is that in different places there are sporadic power outages," Delrish Moss, a Miami-Dade police detective, told CNN.

The utility company was carrying out rolling outages at road intersections, trying to keep traffic moving, Moss said.

He said "a few minutes later the lights come on and we move to another spot. We're seeing some slowdowns but for the most part we're not seeing major problems," Moss said.

Florida Power and Light operates nuclear units in Florida at two sites, Turkey Point and St. Lucie.

Some schools were staying open and not letting students leave as authorities in Miami-Dade county urging citizens to drive cautiously and "stay calm."

"There is no evacuation plan taking place around the area because it's a power problem caused from mechanical failure in the Florida Power and Light system," Mike Stone from the state's emergency department told AFP.

Stone could not confirm the reports of problems at the Turkey Point nuclear plant.

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