JACKSONVILLE (AFP) - Shelters along the eastern coast of Florida were jammed on Thursday (Oct 6) amid a frantic race to protect people and pets from the "potentially catastrophic" effects of Hurricane Matthew as the deadly storm barrels in from the Caribbean.

The largest shelter in the quaint beach city of St. Augustine - founded in the 16th century by Spanish explorers - had reached its capacity of 500 people, and authorities were refusing entry to frustrated residents who were turned back into the rain, pillows under their arms.

"They started coming yesterday afternoon," said Mr Clay Carmichael, principal of Pedro Menendez High School, a stormproof building just inland from the coastal areas facing the most dire threat. "We provide security, electricity, food."

On the school's basketball court, 500 storm refugees had laid out their blankets or mats in neat rows.

While some were visibly nervous, others - notably children and seniors - have been taking advantage of the moment to socialise.

Visitor Maria Maldonado, an 86-year-old Puerto Rican, chuckled as she recounted how she had come to Florida for the baptism of her great-granddaughter and is now stranded, in a gym, on a camp chair.

"Not the perfect vacation," she smiled, "but they are treating us well."

Some three million Americans have been told to evacuate the states of Florida, Georgia and South Carolina. Of these, about 1.5 million residents of coastal Florida were ordered to move to safer locales inland.

Gas stations along the coast have run dry, hotels are turning people away, supermarkets have no more batteries, flashlights or other supplies, and just to buy canned food has become a daunting challenge.

High winds are already whipping resort towns like Jacksonville Beach and Atlantic Beach, and the wind and rain are severely limiting visibility. Cities are deserted, except for a few brave - or foolish - people taking selfies on the beach before the pounding waves.

After surging through the Caribbean, Matthew was arriving late Thursday in Palm Beach, Florida - 100km north of Miami , according to the National Hurricane Center (NHC).

Its impact on Florida will be "potentially catastrophic," with storm surges, extreme winds and heavy rains.

In addition, the NHC warned that residents of high-rise buildings are at particular risk of strong winds.

The St. Johns River, which flows between Jacksonville and St. Augustine, could crest at 6.5 to 10 feet above normal levels, the weather service said. Nearly the entire area is under an evacuation order, with shelters provided for those unable to leave.

Canteens feeding the homeless have opened their doors, and authorities provided school buses to pick up residents without transportation.

The Timberly Creek animal shelter is caring for about a hundred dogs and half as many cats, many of them nervously barking or meowing. The odour is overpowering, the air humid and dense, the noise loud and unremitting.

"Yeah, I know, it's crazy," shouted Mr Nat Walters, of the St. Johns County animal control office, taking a brief break to speak to AFP. "This is one of two animal shelters in the county, and you can't come here if you don't have a pet."

A 55-year-old woman arrived in a wheelchair, accompanied by a small dog on a leash. She would not give her name, but said her husband had died only days ago.

"This is the first time I'm by myself in a situation like this," she said, sobbing, but adding, "I appreciate the service here."

Meantime, in the school gym where humans were sheltering, people were passing the time lying on their mats.

"This sucks," said Dina Capuano. "We should have gone to a hotel," but there were no rooms left.

Her nine-year-old son, at least, was spending time playing cards with his grandmother - a game both appeared to enjoy.