Emergency workers neared the end of their search Tuesday afternoon for survivors in Moore, Okla., following a deadly tornado that weather officials said was now classified among the most powerful type of twister.

The tornado, which levelled two elementary schools and many homes, killed 24 people, including nine children.

Fire Chief Gary Bird said that he's "98 per cent sure" there are no more survivors or bodies to recover under the rubble in Moore, Okla. He said every damaged home had been searched at least once and that he's hopeful the work could be completed by nightfall Tuesday, though heavy rains slowed efforts.

Bird said no additional survivors or bodies have been found since Monday night.

The number of deaths could rise, Gov. Mary Fallin indicated Tuesday, saying there have been 237 injuries reported in the aftermath of the storm 16 kilometres south of Oklahoma City.

The National Weather Service confirmed the tornado ranked as an EF-5, which is the most powerful type of twister on the enhanced Fujita scale. An EF-5 packs sustained winds of more than 322 km/h.

Speaking at a televised news conference, Fallin said the unified command centre has received reports that some bodies may have been taken directly to funeral homes. She thanked federal officials for their help and support and singled out first responders who searched valiantly for survivors in the rubble.

Earlier, U.S. President Barack Obama assured the country that Oklahoma would receive federal emergency aid to help with the recovery, and he reached out especially to those whose children were killed.

"In some cases, there will be enormous grief that has to be absorbed, but you will not travel that path alone," the president said in a televised address. "Your country will travel it with you, fuelled by our faith in the Almighty and our faith in one another."

Canada's Foreign Affairs Minister John Baird released a statement extending his condolences to the U.S.

"While we are confident in the abilities of emergency personnel on the ground and sure of the resilience of local people, Canada remains ready to provide assistance if needed," he said.

Initial reports that the storm had killed 51 people or more were scaled back Tuesday morning. Officials in Oklahoma City said that 24 bodies had been recovered after the tornado, nine of them children.

The tornado demolished Briarwood and Plaza Towers elementary schools in Moore, reducing homes to piles of splintered wood.

"It would be incredible if anybody survived in any of those structures during this terrible storm," Fallin told CBS News on Tuesday morning, as Moore, a town of 41,000, braced for another harrowing day.

10 deadliest U.S. tornadoes

 1.      March 18, 1925: Mo., Ill., and Ind., 695 killed.

 2.      May 6, 1840: Natchez, Miss., 317 killed.

 3.      May 27, 1896: St. Louis, Mo., 255 killed.

 4.      April 5, 1936: Tupelo, Miss., 216 killed.

 5.      April 6, 1936: Gainsville, Ga., 203 killed.

 6.      April 9, 1947: Woodward, Okla., 181 killed.

 7.      May 22, 2011: Joplin, Mo., 158 killed.

 8.      April 24, 1908: Amite, La., and Purvis, Miss., 143 killed.

 9.      June 12, 1899: New Richmond, Wis., 117 killed.

 10.   June 8, 1953: Flint, Mich., 116 killed.

 Source: U.S. National Weather Service/NOAA

Moore resident Joey Wallace, his seven-month-old son, his two dogs and three of his neighbours survived the deadly tornado by hiding in his storm cellar.

"It went by pretty fast," Wallace told CBC's As It Happens, explaining the tornado passed over his house in about 30 seconds and left "nothing" of his home intact.

"You felt [the tornado] trying to pull the door open," he said, adding he kept thinking it would happen.

After the storm passed and Wallace slid open the storm cellar door, he said the scene looked "almost like a bomb went off."

Fallin said at an afternoon press conference that authorities were working hard to achieve an accurate count of those killed by the storm.

She said Oklahoma Gas and Electric put the number of people without power at 38,000 — 20,000 of them in Moore and Oklahoma City. As well, AT&T and Verizon had set up mobile units to improve wireless communication.

Federal Emergency Management Agency administrator Craig Fugate urged people who need help to get in touch with the agency at www.disasterassistance.gov or by calling 1-800-621-FEMA (3362).

Rescue shifts to recovery

Earlier Tuesday, rescue efforts appeared to have become devoted more to recovery.

"Some folks [are] saying this is more of a recovery mission at this stage as the hours and minutes go by," CBC News correspondent Lyndsay Duncombe reported. "Of course, it becomes less likely that survivors will be found and more likely that that death toll will go up."

More than 120 people, including about 50 children,  were being treated at hospitals. The fatalities among children were located at Plaza Towers Elementary, rather than Briarwood, CBS News reported.

"We have seen some of those images of just parents and volunteers who were rushing to the school to start pulling some of those children out, so anyone who could help did," Duncombe reported. "Now they're asking people to stay away from the scene and let the professionals do their work."

Children from Plaza Towers school were among the dead, but several students were pulled out alive earlier Monday from under a collapsed wall and other heaps of mangled debris. Rescue workers passed the survivors down a human chain of parents and neighbourhood volunteers.

Parents carried children in their arms to a triage centre in the parking lot. Some of the students looked dazed, while others appeared terrified.

As dusk fell, heavy equipment rolled up to the school and emergency workers wearing yellow crawled among the ruins, searching for survivors. Crews used jackhammers and sledgehammers to tear away concrete, and chunks were being thrown to the side as the workers dug.

The governor deployed 80 National Guard members to assist with rescue operations and activated extra highway patrol officers.

Fallin also spoke Monday with Obama, who declared a major disaster and ordered federal aid to supplement state and local recovery efforts.

In video of the storm, which touched down at 2:56 p.m. local time and lasted about 40 minutes, the dark funnel cloud could be seen marching slowly across the green landscape. As it churned through the community, the twister scattered shards of wood, awnings and glass all over the streets.  

The tornado also destroyed the community hospital and some retail stores.

A map provided by the National Weather Service at Norman, Okla., showed that the storm began west of Newcastle and crossed the Canadian River into Oklahoma City's rural far southwestern side. When it reached Moore, the twister cut a path through the centre of town before lifting back into the sky at Lake Stanley Draper.

The National Weather Service issued an initial finding that the tornado was an EF-4 on the enhanced Fujita scale, the second-most powerful type of twister. By Tuesday afternoon, the tornado was upgraded to an EF-5.

CBC Newfoundland meteorologist Ryan Snoddon, who happened to be in Oklahoma on the weekend with a storm-chasing crew south of Moore, said he had never seen anything like the "debris ball" visible on radar.

"Typically, what you see coming back is rain," he told CBC News on Tuesday morning. "It wasn't rain — it was just a debris cloud, and … the scale of it, the  size of it, you know, it was like every time the radar made a scan, the storm just  kept getting stronger.

"You almost wish you could just pick it up and move it, because it was really a worst-case scenario where it was tracking into the Moore area." 

With files from The Associated Press and Reuters