OKLAHOMA CITY -- Electric company workers made some progress early Wednesday in an effort to restore power to more than 634,000 customers, and officials said Wednesday would be a telling and possible dangerous day.

Officials estimate that as many as 1.5 million people were without power on Tuesday, the result of a weekend ice storm. According to federal figures, Oklahoma's 2006 population was 3,579,212, meaning that roughly 42 percent of the state was without power, two days after an ice storm blanketed the state.

State emergency management officials said early Wednesday that just under 498,000 homes and business remained without power in what is the largest outage in Oklahoma history. Power crews have come from Texas, New Mexico, Louisiana and Indiana to help Oklahoma Gas & Electric workers restore power.

They said they know Wednesday will be difficult and could be dangerous.

Officials with OG&E said they are still anticipating seven to 10 days to restore power to their 294,000 customers without electricity. It could be longer for some customers, according to OG&E spokesman Brian Alford.

Alford said it was fortunate that they hadn't lost a great number of electricity poles; however, he added that a lot of power lines were on the ground.

A mixture of snow, sleet and light rain has been falling Wednesday morning across parts of Texas and Oklahoma, where it could be up to 10 days before power is fully restored.

Ice more than an inch thick has glazed much of the affected area this week. At least 25 deaths -- mostly from traffic accidents -- are being blamed on the storm system since it developed last weekend.

Elsewhere, forecasters say more snow, sleet and freezing rain could develop Wednesday across the northern Ohio Valley and into New England.

Millions of utility customers were struggling without electricity in the nation's midsection after a massive storm dropped sleet and freezing rain across much of Kansas, Missouri, Illinois and Iowa.

OG&E has requested that those who lose power report the outage; however, they have asked that customers who have reported an outage not call again. They also request that people do not call them to ask when power will be restored.

At this point, officials said, they don't know.

The storm also caused extensive travel problems. About 560 flights were canceled at Chicago's O'Hare Airport, and hundreds of other flights were badly delayed.

In Oklahoma, schools were closed for a second day across most of the state. Classes were also canceled in Kansas, Iowa and Wisconsin, including the Milwaukee district, with 85,000 students.

Officials in Missouri, Kansas and Oklahoma had declared states of emergency. President George W. Bush declared a federal emergency in Oklahoma on Tuesday, ordering government aid to supplement state and local efforts.

At least 24 deaths in the Midwest, 15 in Oklahoma, have been blamed on frozen conditions that sent cars skidding off highways and caused trees and power lines to snap under the weight of ice.

Tulsa and Oklahoma City each had more than 100 reports of fires since the storm began, mostly from tree limbs crashing into live power lines, authorities said.

Until Tuesday, the volunteer fire department in the small Kansas town of Durham had gone on just two fire runs all year. Within hours, the department rushed to the scene of three weather-related electrical fires.

"I don't know as we've ever run that many fires," said Fire Chief Pete Sommerfeld, who was without power along with the rest of the town of 110 people.

The Federal Emergency Management Agency rushed 50 industrial generators to Oklahoma for hospitals, water-treatment plants and emergency shelters, and 50 more were on the way. FEMA was also providing blankets, cots and pre-packaged meals.

Des Moines television station KCCI reported that extra repair crews have been brought in to Iowa from across the Midwest before the storm hit early Tuesday morning.

Power and road crews worked overnight into Wednesday morning to help repair the damage, but more than 25,000 power customers are estimated to be without power. Utility officials said they are working as fast as possible to restore power, but it could take 24-48 hours for crews to complete their work.