By Steve Olafson

OKLAHOMA CITY (Reuters) - Cooler temperatures gave a slight boost on Sunday to crews battling 18 wildfires across Oklahoma that have torched at least 121 structures and charred thousands of acres amid a drought.

The blazes, which have charred more than 68,000 acres across the state, burned throughout the night, forcing more evacuations in small towns and rural areas in central Oklahoma, although it will be days before the full damage is tallied, officials said.

One of the most devastating fires tore through the town of Luther, about 25 miles from Oklahoma City, on Saturday, razing 56 structures, although it is now under control. Authorities are investigating whether that blaze was deliberately set.

A cool front that passed across the state overnight lowered temperatures by up to 25 degrees Fahrenheit and may keep the mercury from topping the 100-degree F (38 degrees Celsius) mark in areas where fire crews are working on Sunday, the National Weather Service said, although underlying drought conditions remain unchanged.

"This year is a bad year, but it's not as bad as last year -- only because we got a late start," said Michelle Finch Walker, a spokeswoman for Oklahoma Forestry Services. "This year is going to be remembered for the number of structures lost," she added.

The largest of the state's fires has blackened a 58,000-acre (23,490 hectare) stretch between Oklahoma City and Tulsa, the state's two largest cities, Finch said, noting that it was one of the largest blazes in the state's history.

No deaths have been reported. Oklahoma remains under a state of emergency, with a ban on outdoor burning by order of Governor Mary Fallin.

Oklahoma has joined several other states including Colorado, Utah, Arizona, Arkansas and Nebraska in being struck by wildfires during the widespread drought.

Nearly two-thirds of the contiguous United States was experiencing some level of drought as of July 31, according to the Drought Monitor, a weekly report compiled by U.S. climate experts. Nearly all of Oklahoma was under severe drought or worse.

Below-normal rainfall, temperatures up to 113 degrees Fahrenheit (45 Celsius) and wilted vegetation have made the potential for wildfires extremely high throughout Oklahoma.

(Editing by Tim Gaynor and Cynthia Osterman)


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