At least four people have died after a storm swept across the southern United States, knocking out electricity to more than 600,000 homes and facilities.

The storm weakened yesterday from a hurricane to a tropical or post-tropical storm after making landfall in Florida, bringing heavy rain and strong winds, according to the US national hurricane centre.

Several states on the Gulf of Mexico coast, including Florida and Alabama, declared a state of emergency and ordered the evacuation of areas threatened by violent winds, high waves and torrential rain.

Winds accompanying the storm reached 165 kilometres per hour. The impact also hit the energy sector, with about 63% of oil production and 57% of gas production in the Gulf of Mexico suspended.