Miami - Hurricane Irma slammed into the Florida Keys Sunday, lashing the tropical island chain with fearsome wind gusts as it churns towards the US state's west coast where a mass exodus has turned cities into ghost towns.Six million people - one third of the state's population - have been ordered to evacuate the path of the monster hurricane, which was upgraded to a Category Four storm as it passed over the Keys packing maximum sustained winds of 215 km/h.In homes, hotels and school gyms, a die-hard minority who defied orders to flee were hunkered down as Irma's roaring winds ripped boats from their moorings, flattened palm trees and tore down power lines across the island chain popular for fishing and scuba diving."There's absolutely no way anybody can be outside right now," Maggy Howes, a first responder on Key Haven, said on CNN. "You would not be able to stand or walk."As of 11:00 (15:00 GMT), Irma's eye was located around 120km south-southeast of Naples, on Florida's southwest coast, and slowly moving away from the Keys.One of the most powerful hurricanes ever to slam storm-prone Florida, Irma is threatening dangerous storm surges up to 4.5 m, enough to cover a house, as it collides with the state after sowing devastation through the Caribbean.A shelter of last resort set up in the Middle Keys city of Marathon was reported to be without power or running water, and surrounded by surging waters."Everything is underwater, I mean everything," Larry Kahn, an editor for local news website FlKeysNews.com, reported from inside.The cities of Naples, Fort Myers and the densely populated peninsulas of Tampa Bay on Florida's west coast are next in the crosshairs of the historic storm, churning north at nine miles per hour and already lashing the mainland with dangerous winds."It's going to be horrible," Florida Governor Rick Scott said on NBC Sunday morning. "Now we have to hunker down and watch out for each other." Tampa Mayor Bob Buckhorn was more blunt: "We are about to get punched in the face by this storm."Mother delivers baby alone Irma smacked the Keys 57 years to the day that Hurricane Donna hit the same area in 1960, destroying nearly 75% of the island chain's buildings.On the mainland, emergency services in Miami were sheltering in place from winds of up to 120km/h , and a dispatcher talked a woman through delivering her own baby on Sunday morning, Assistant Fire Chief Eloy Garcia told the Miami Herald.More than one million Florida homes and businesses were without power, according to utility company Florida Power and Light, which said it had "safely shut down" one of two nuclear reactors at its Turkey Point power plant.The National Weather Service urged Floridians to keep their shoes on, to take shelter in interior rooms - far from windows - and use helmets, mattresses, pillows or blankets for protection.At least 27 people have been killed since Irma began its march through the Caribbean, smashing through a string of islands from tiny Barbuda on Wednesday, to the tropical paradises of St Barts and St Martin, the US Virgin Islands, Puerto Rico, the Dominican Republic, Haiti and the Turks and Caicos.Terrified Cubans who rode out Irma in coastal towns - after it made landfall Friday on the Camaguey archipelago as a maximum-strength Category Five storm - reported "deafening" winds, uprooted trees and power lines, and blown rooftops.There were no immediate reports of casualties in Cuba but it caused "significant damage," and enormous waves lashed the Malecon, Havana's emblematic seafront, causing seawaters to penetrate deep into the capital, AFP journalists reported.Storm surge, tornado risk Irma is so wide that authorities were bracing for destructive storm surges on both coasts of Florida and the Keys as Irma follows a path north toward Georgia.The NHC also warned of tornado risks through Sunday night, with the greatest threat lying in areas east of the storm's path.Strip malls, fast food restaurants and retail giants were closed for business on both of Florida's coasts.MacDill Air Force Base in Tampa, the military installation home to US Central Command, issued mandatory evacuation orders ahead of the storm's passage early on Monday, while the Kennedy Space Center on the east coast was also closed.In Naples, the city's chic coastal neighbourhoods stood deserted on Sunday morning, as torrential rain beat down on streets littered with leaves ripped from palm trees.But Viviana Sierra, who sought refuge at a shelter outside the city together with her dog, parents and brother, was sanguine about the prospect of finding her home destroyed. "You can replace material things but your life is very important, so I think it's better that we stay here," she said.