Hurricane Irma lost some strength as it pounded southern Florida on Sunday afternoon, but forecasts warned it would remain a powerful storm as it flooded Miami streets and knocked out power to about 2 million homes and businesses.

All of southern Florida was feeling the effects of the storm creeping towards the shore, with at least one man killed, a woman forced to deliver her own baby, apartment towers swaying in high winds and trees uprooted.

The National Hurricane Center said the storm had maximum sustained winds of 120 miles per hour (195 kph), dropping it to a Category 3, the midpoint of the five-step Saffir-Simpson scale.

Irma had been one of the most powerful hurricanes ever seen in the Atlantic, killing 28 people in the Caribbean and pummeling Cuba with 36-foot (11 meter) waves on Sunday.

Some 6.5 million people, about a third of the state's population, had been ordered to evacuate southern Florida.

"This is a life-threatening situation," Governor Rick Scott told a press conference according to Reuters.

Tornadoes were also spotted through the region.

Irma is expected to cause billions of dollars in damage to the third-most-populous U.S. state, a major tourism hub with an economy that generates about 5 percent of U.S. gross domestic product.

About 2 million Florida homes and businesses had lost power, according to Florida Power.

The storm killed 24 as it raged through the Caribbean. It has already claimed at least one life in Florida, a man found dead in his pickup truck, which had crashed into a tree in high winds.

Irma comes just days after Hurricane Harvey dumped record-setting rain in Texas, causing unprecedented flooding, killing at least 60 people and an estimated $180 billion in property damage. Almost three months remain in the Atlantic hurricane season, which runs through November.

U.S. President Donald Trump spoke to the governors of Alabama, Georgia, South Carolina and Tennessee on Sunday and issued a disaster declaration for Puerto Rico, which was hit by the storm last week, the White House said.