UPDATE 6-Rescuers scour debris for dead after Florida hurricane

(Adds comments from Senator Nelson, CNN reporter, survivor)

MEXICO BEACH, Fla., Oct 12 (Reuters) - Rescue crews worked their way through fallen trees and heaps of rubble on Friday in the Florida Panhandle towns hit the hardest by Hurricane Michael, looking for people trapped or killed by a storm blamed for at least 12 deaths.

The concern was for people who ignored evacuation orders ahead of the storm - which grew with surprising speed from a tropical storm into an extremely powerful hurricane in less than two days - and who stayed put in communities that were demolished by Michael's assault on Wednesday.

"I think youre going to see it climb," Brock Long, administrator of the Federal Emergency Management Agency, said of the death count at a news conference. "We still havent gotten into some of the hardest-hit areas."

FEMA crews used heavy equipment, sniffer dogs, drone aircraft and global positioning satellites in their search.

So far, counties along the affected northwest Florida coast have reported no deaths related to the storm.

Michael charged ashore near the small Florida Panhandle town of Mexico Beach as one of the most powerful storms in U.S. history, with winds of up to 155 miles per hour (250 km per hour). It pushed a wall of seawater inland, causing widespread flooding.

Many houses in Mexico Beach were reduced to naked concrete foundations or piles of rubble. The storm, a Category 4 on the five-step Saffir-Simpson scale, tore apart entire neighborhoods in the Panhandle.

Phone service to the most damaged areas was down, leaving survivors no way to contact worried relatives, so a CNN reporter on the air read off several names of residents he had spoken with, to let concerned parties know they were safe.

It was rough," Mary Grasberger 50, said of her ordeal after riding out the storm in a motel in Panacea, a community near the Gulf of Mexico coast. "We were surrounded by water and we could see road signs floating by. It was crazy. Two of the light poles were knocked down by the winds.

"Thank God the rooms didnt flood."

Florida Senator Bill Nelson went to Panama City, up the coast from Mexico Beach, and was stunned by what he saw.

"Pine forests were no longer - they were all sticks that were broken in two," Nelson, a Democrat, told Fox News. "Mexico Beach, there's no barrier island out there to protect it so it got the full force of the Category 4 wind and that wall of water."

DEATHS ACROSS THE SOUTHEAST

Although weaker as it pushed over the southeastern United States, the storm carried high winds and delivered drenching rains to Georgia, the Carolinas and Virginia. It killed at least 12 people in Florida, Georgia, North Carolina and Virginia, officials said.

In Virginia, the remnants of the hurricane swept away four people in floodwaters. A firefighter also was killed when hit by a truck as he was trying to help an accident victim, the Washington Post reported.

About 1.5 million homes and businesses were without power from Florida to Virginia early on Friday, according to utility companies.

It could be weeks before power is restored to the most damaged parts of Florida.

Long urged communities such as Mexico Beach, where many homes were obliterated by 12 to 14 feet (3.7 to 4.3 meters) of storm surge, to rebuild to withstand future storms.

"Its OK if you want to live on the coast or on top of a mountain that sees wildfires or whatever but you have to build to a higher standard," he said. "If were going to rebuild, do it right."

By Friday morning the remnants of Michael had moved into the Atlantic Ocean northeast of Norfolk, Virginia, but still could bring up to 5 inches (13 cm) of rain to parts of New England, the National Hurricane Center said.

The number of people in emergency shelters was expected to swell to 20,000 across five states by Friday, said Brad Kieserman of the American Red Cross. The Coast Guard reported rescuing 129 people.

Michael severely damaged cotton, timber, pecan and peanut crops, causing estimated liabilities as high as $1.9 billion and affecting up to 3.7 million crop acres (1.5 million hectares), said Brad Rippey, a meteorologist for the U.S. Agriculture Department.

The storm also disrupted energy operations in the U.S. Gulf of Mexico as it approached land, cutting crude oil production by more than 40 percent and natural gas output by nearly a third as offshore platforms were evacuated.

(Reporting by Rod Nickel in Mexico Beach, Fla. Additional reporting by Devika Krishna Kumar in Port St. Joe, Fla., Gina Cherelus and Scott DiSavino in New York, Gary McWilliams and Liz Hampton in Houston, Jon Herskovitz in Austin, Texas, Brendan O'Brien in Milwaukee and Alex Dobuzinskis and Dan Whitcomb in Los Angeles Writing by Dan Whitcomb and Bill Trott Editing by Frances Kerry)