Two big rigs and two passenger vehicles collided and spilled diesel fuel across a Florida highway on Thursday, sparking a massive fire that killed seven people, authorities said.

The wreck happened on Interstate 75 about 1.6 km south of Alachua, near Gainesville. The flames were fed by about 189 litres of diesel, authorities said.

Several others were taken to the hospital, some with critical injuries, the Gainesville Sun reported. Authorities initially said six had died but late on Thursday night revealed a seventh victim had perished.

Emergency crews extinguished the fire and said they were treating the crash as a homicide investigation, but didn't say why. The fire was so intense that authorities said it damaged parts of the road.

A spokesperson for the Florida Highway Patrol told The Associated Press in a phone interview that their top priorities were to conduct a thorough investigation and to identify the deceased victims.

'There's going to be families that need to be notified that their loves ones have perished,' said Lieutenant Patrick Riordan.

It's unclear whether the victims were killed in the wreck or whether they burned in the fire, which would make identification more difficult, he said.

The aftermath closed part of the highway in both directions, causing massive delays.

The crash was in the northbound lanes, but southbound lanes were closed for hours to keep a route open for first responders, according to a tweet from the Alachua County Sherriff's office, which said the emergency 'required all hands on deck.' Authorities opened the northbound lanes around 8 pm but said southbound lanes could be closed until morning.

Debris including personal property and vehicle parts was scattered across the road, the Florida Highway Patrol said.

A helicopter arrived to search for any victims who may have been in nearby woods.