Tropical Storm Debby drenched Florida on Monday, killing at least four people and threatening southeastern US states with heavy rainfall and catastrophic flooding.

A 13-year-old boy died when a tree was blown onto a mobile home in Levy County, the sheriff's office there said, after Debby made landfall on Florida's Gulf Coast earlier Monday as a Category One hurricane.

Authorities said a truck driver was killed after his 18-wheeler plunged into a canal in Hillsborough County, while a 38-year-old woman and 12-year-old boy died in a car crash in Dixie County.

The storm moved into Georgia overnight and is expected to go offshore before approaching the South Carolina coast on Thursday, according to the National Hurricane Center (NHC).

"This is a level four out of four risk for excessive rainfall," Michael Brennan, director of the NHC, told reporters.

"This is going to result in a prolonged extreme rainfall event with potential for catastrophic flooding across coastal portions of Georgia, South Carolina, even extending up into North Carolina," he added.

Florida Governor Ron DeSantis said that some 250,000 residents in his state were without power.

"Please, be very cautious when you're going out," he said, adding that Debby's winds had not been as damaging as previous hurricanes that have hit Florida.

President Joe Biden has approved emergency declarations for Florida, Georgia and South Carolina, allowing for federal assistance in coordinating disaster relief efforts.

DeSantis also activated his state's National Guard, with more than 3,000 service members mobilized to help with storm response.

Evacuation orders

By Monday evening, the NHC said the storm was registering maximum sustained winds of 45 mph (75 kph) as it swept into Georgia.

Storm surge warnings -- signalling a life-threatening inundation from rising water -- are in effect in parts of Georgia, South Carolina and North Carolina.

Debby was expected to bring "potentially historic rainfall" of up to 30 inches as it moved north, the NHC said.

But it said the storm was weakening after making landfall earlier with sustained speeds of 80 mph (130 kph) as a Category One hurricane -- the lowest on a scale of five.

Mandatory evacuations were ordered for part of Citrus County, Florida, with several other counties under voluntary evacuation orders, local media reported.

The governors of Georgia and South Carolina declared a state of emergency ahead of Debby's arrival.

Vice President Kamala Harris -- the Democratic candidate in the US election -- postponed events in North Carolina and Georgia this week due to the storm, according to local media reports citing her campaign team.

Meanwhile, the US Border Patrol announced that Debby had washed up 25 packages of cocaine worth around $1 million to the coast of the Florida Keys, where they were seized.

In July, at least 18 people were killed when the powerful Hurricane Beryl tore through the Caribbean before hitting the southern US states of Texas and Louisiana.

Scientists say climate change likely plays a role in the rapid intensification of storms such as Beryl because there is more energy in a warmer ocean for them to feed on.

© Agence France-Presse