Powerful Hurricane Ian left a trail of destruction and caused a widespread blackout in Cuba on Tuesday, while Florida braced for a direct hit from the "extremely dangerous" storm that is already pummeling the US state with high winds.

Ian hit Cuba's western regions for more than five hours early Tuesday morning, before moving out over the warm waters of the Gulf of Mexico, the Insmet meteorological institute said.

The storm damaged Cuba's power network and plunged the island into darkness, leaving it "without electrical service," state electricity company Union Electrica said on Twitter.

In the western city of Pinar del Rio, videos showed downed power lines, flooded streets and a scattering of damaged rooftops.

"Desolation and destruction. These are terrifying hours. Nothing is left here," a 70-year-old resident of the city was quoted as saying in a social media post by his journalist son, Lazaro Manuel Alonso.

About 40,000 people were evacuated across Pinar del Rio province, which bore the brunt of the storm, local authorities said.

The US National Hurricane Center (NHC) said it expects Ian to gain strength before hitting the west coast of Florida as an "extremely dangerous" major hurricane.

Calls to heed evacuation warnings were echoed from local Florida officials on up to US President Joe Biden, who said Ian "could be a very severe hurricane, life threatening and devastating in its impact".

In its latest bulletin, the NHC said to be prepared for "life-threatening storm surge, catastrophic winds and flooding" in the Florida peninsula.

The Caribbean and parts of eastern Canada are still counting the cost of powerful storm Fiona, which tore through last week, claiming several lives.

Half a million residents in the US territory of Puerto Rico were still without power, according to a tracking website. (AFP)