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At least 51 people were killed, including 20 children, after a massive tornado roared through Oklahoma City on Monday afternoon, flattening entire neighborhoods and crushing two elementary schools.

Spokeswoman Amy Elliott told The Associated Press early Tuesday that officials could see as many as 40 more deaths from the Monday's twister.

President Obama declared a major disaster in Oklahoma late Monday as search-and-rescue efforts continued throughout the night.

More than 120 people were being treated at hospitals, including about 50 children, after the ferocious storm ripped through the suburb of Moore in a Midwest region of the U.S. known as Tornado Alley.

Catastrophic damage was reported in Moore, which was flattened by another killer tornado that tracked the same path 14 years ago.

All around Moore on Monday night, grim-faced police officers and sheriff's deputies guarded darkened intersections, their boots crunching on the debris littering the ground.

Several children were pulled alive from the rubble of Plaza Towers Elementary, but some of their classmates were killed. About a mile away, the walls tumbled down at Briarwood Elementary. Miraculously, no one there died.

KFOR-TV reported that seven of the dead were children from Plaza Towers, where 75 students and staff members were huddled when the tornado struck. U.S. Rep. Tom Cole, who has lived in Moore for more than 50 years, told CNN the school did not have an underground shelter, just interior rooms with no windows.

At Integris Southwest Medical Center, 10 of 37 patients were in critical condition, a spokeswoman told the Associated Press. The hospital was treating five children, including two rescued from the elementary school. The OU Medical Center was treating 20 patients, including eight children.

More than 60 patients were being treated at Norman Regional Medical Center, some in critical condition, spokeswoman Kelly Wells said.

One patient was 9-year-old Kaileigh Hawkins, who was at one of the schools destroyed by the twister, Wells said. She is doing fine, but hospital officials have been unable to locate her parents.

The twister heavily damaged Moore Medical Center, ripping off its roof but causing no injuries. Staff had to relocate 30 patients to nearby Norman and another hospital.

A water treatment was knocked offline, and residents and businesses in southeastern Oklahoma City were advised to stop using water.

The preliminary rating of the tornado that hit Moore at 3:17 p.m. CT (4:17 p.m. ET) was put at EF-4, which means wind speeds from 166 to 200 mph, the National Weather Service said.

On May 3, 1999, a record-setting EF-5 tornado obliterated the city of 55,000 with winds measured at 318 mph, the highest ever on the earth's surface. The storm killed 36 people, injured hundreds and caused about $1 billion in damages.

The National Weather Service in Norman, Okla., said a tornado warning was in effect Monday afternoon for 16 minutes before the twister developed.

Rescuers were "going house to house and block to block to try and find any survivors that are out there and trapped,'' said state emergency management spokesman Jerry Lojka.

"We can only imagine that there are still many others there that are unaccounted for,'' he said.

Lojka said emergency management officials were working from an underground command center in Oklahoma City and did not yet know how many students were in the two elementary schools in Moore that were destroyed.

He said the funnel cut a path one and a quarter miles wide, traveling the entire width of the city.

"It went through an area and demolished complete subdivisions,'' he said. "It affected shopping centers, a theater, two schools that we know of.''

Officials said Monday evening that they would work all night to find survivors.

"Search-and-rescue efforts will continue through the night, as long as they need be," Moore Police Chief Jerry Stillings said at a news conference.

"We've been through this before,'' city manager Steve Eddy said, referring to the 1999 tornado and others over the years. "Our citizens are resilient.

Oklahoma Gov. Mary Fallin called up 204 Oklahoma National Guard personnel. She also spoke with President Obama, who offered her a direct line to his office and federal aid. She said she had been in touch with the Federal Emergency Management Agency.

Fallin had earlier declared emergencies for 16 Oklahoma counties, adding five more after the tornado roared roared through the Oklahoma City area and continued to the northeast.

Oklahomans who escaped the destruction counted their blessings and offered to help others less fortunate.

Holly Porter and her husband, Tracy, loaded up their truck with water and blankets, hitched up their horse trailer and set out on for what normally is a 40-minute drive to Moore, where their son and daughter-in-law live.

Because the tornado sucked away barns and fencing, she said, residents may have nowhere to keep their animals.

"We have 60 acres and we can take cows and horses," she said. "We're just going to help anyone we can."

The Porters had several anxious minutes when they couldn't reach their daughter-in-law, a substitute teacher at an elementary school. Eventually, Holly Porter got through and learned that their daughter-in-law's school had not been hit and that all the children were fine.

At Plaza Towers Elementary, end-of-the-year festivities that began last week continued Monday. First- and second-graders got their awards, with third-, fourth- and fifth-grade students scheduled to collect their honors Tuesday morning after "Rise and Shine" activities.

Monday night, the school's PTA Facebook page was awash in condolences and prayers.

Numerous charities have swung into action, delivering food and water for first-responders. Oklahoma Gov. Mary Fallin visited the Plaza Towers school late Monday night, thanking them in person and via Twitter for "their hard work and tireless dedication."

On Sunday, a tornado packing winds as high as 200 mph, left two people dead in Oklahoma. Tornadoes and high winds injured more than 20 in the region.

So far this year ? not including this most recent five-day outbreak ? severe storms have caused $3.5 billion in economic losses in the USA, said meteorologist Steve Bowen of global reinsurance firm Aon Benfield. Bowen says. Of that $3.5 billion, at least $2 billion was covered by insurance.

"By the time the current storm system finally winds down by the middle of this week, I wouldn't be surprised if this ends up as the costliest U.S. natural disaster event we've seen so far in 2013," said meteorologist Steve Bowen of global reinsurance firm Aon Benfield.

"Recent full-year severe weather-related insured losses were roughly $27 billion in 2011 and $15 billion in 2012 ? the two costliest years on record," Bowen said. By this definition, "severe" weather means damage from thunderstorms or tornadoes, and does not include damage from hurricanes.

A tornado touched down in Golden City, Mo., early Monday and tore through two counties, Barton County Emergency Management Director Tom Ryan told CNN.

Meteorologist Kurt Kotenberg said a large low-pressure system is parking itself over the middle of the country and "really isn't going to move much over the course of the next few days. ... It's basically going to keep pulling up that nice Gulf (of Mexico) moisture that keeps fueling everything."

The threat of twisters comes less than a week after tornadoes left six dead, dozens injured and hundreds of homes destroyed in Texas and just shy of the two-year anniversary of the Joplin, Mo., twister.

Contributing: Susan Davis, Doug Stanglin, USA TODAY; Trevor Hughes, The Coloradoan; The Associated Press