A wintry storm caked the center of the nation with a thick layer of ice Monday, blacking out more than 600,000 homes and businesses, and more icy weather was on the way. At least 18 deaths in Oklahoma and Missouri were blamed on the conditions, with 15 of them killed on slick highways. A state of emergency was declared for all of Oklahoma, where the sound of branches snapping under the weight of the ice echoed through Oklahoma City. "You can hear them falling everywhere," Lonnie Compton said Monday as he shoveled ice off his driveway. The National Weather Service posted ice and winter storm warnings Tuesday for parts of Missouri, Kansas, Nebraska, Iowa and Illinois. Missouri declared an emergency on Sunday and put the National Guard on alert. Oklahoma utilities said a half-million customers were blacked out as power lines snapped under the weight of ice and falling tree branches, the biggest power outage in state history, and utilities in Missouri said more than 100,000 homes and business had no power there. "If you do the math, probably one out of three Oklahomans has no electricity at this point," said Gil Broyles, a spokesman for Oklahoma Gas & Electric, the state's largest utility. Roughly 11,000 customers were blacked out in southern Illinois and more than 5,000 had no electric heat or lights in Kansas, where Gov. Kathleen Sebelius declared a statewide state of emergency. At O'Hare International Airport, about 200 flights were canceled Monday, with delays of about 45 minutes, said Chicago Department of Aviation spokeswoman Karen Pride. Fewer than a dozen flights were canceled at Midway Airport, and a handful of flights were delayed about an hour, she said. Ice was as much as an inch thick on tree limbs and power lines in parts of the region. Schools across Oklahoma were closed and some hospitals were relying on backup power generators. The Federal Emergency Management Agency and the Army Corps of Engineers sent 50 generators and three truckloads of bottled water from Texas to distribute to blacked-out areas of Oklahoma. Tulsa International Airport had no power for about 10 hours and halted flight operations for the day, and most morning flights at Will Rogers World Airport in Oklahoma City were canceled because of icy runways. Greyhound bus passengers were stranded overnight at a shelter in a church in Tulsa, and were joined by some local residents who had no heat. Portions of Interstate 35 and Interstate 44 were shut down early Monday afternoon in Oklahoma City after ice-laden power lines collapsed and fell into the roadways. Oklahoma utility officials said it could be a week or more before power was fully restored. "This is a big one. We've got a massive situation here and it's probably going to be a week to 10 days before we get power on to everybody," said Ed Bettinger, a spokesman for Public Service Company. "It looks like a war zone." The Oklahoma City suburb of Jones, a town of 2,500 people, had low water pressure because there was no electricity to run well pumps, and firefighters said an early morning fire destroyed most of the community's high school. Since the storm began, Tulsa firefighters have responded to dozens of structural fires, most attributable to the storm, said Sheryl Lovelady, a city spokeswoman. One person was killed by smoke inhalation in a storm-related fire, she said; she did not provide details. The icy weather stretched into the Northeast, where many schools across upstate New York were closed or started late because of icy roads. On ice-covered Interstate 40 west of Okemah, Okla., four people died in "one huge cluster of an accident" that involved 11 vehicles, said Highway Patrol Trooper Betsey Randolph. Ten other people died on icy Oklahoma roads. In Missouri, one person died on a slippery highway, while two others died in separate incidents when tree limbs fell on them, authorities said. In addition, a homeless person died of hypothermia in Oklahoma City, the state medical examiner's office said.