Families returned to a blasted moonscape that had been an American suburb Tuesday after a monstrous tornado tore through the outskirts of Oklahoma City, killing at least 24 people.

Passengers flying into Oklahoma City could see the track left by nature's fury as it played out Monday: the spot where the tornado touched down, then chewed through the suburb of Moore like a giant lawnmower for 45 terrifying minutes.

Nine children were among the dead and entire neighborhoods vanished, with often the foundations being the only thing left of what used to be houses and cars tossed like toys and heaped in big piles.

"It's unreal. It's so visceral," said 32-year-old accountant Roger Graham as he combed through the ruins of the three-bedroom home he shared with his wife Kalissa, a school teacher, recovering what he could.

Kelly Pirtle of the US weather agency's Severe Storms Laboratory in nearby Norman, told AFP the tornado was the strongest possible category, EF5, packing winds of more than 200 miles per hour (321 kilometers per hour).

Oklahoma City police chief Bill Citty told a news conference that 20 people had been killed in Moore and four more elsewhere in the city.

"There could be obviously others in the coming days. The search is still going on, heavily in Moore because they have such a large area to cover," he said, adding: "So we have a 24 right now. There could be more.

"All of the people that have been reported missing -- initially last night, about 48 -- all of those have been actually found except for I think a few left in Moore that they are working on to try to locate that have not."

At least 101 people have been pulled alive from under debris, said Terri Watkins of the Oklahoma Department of Emergency Management, and officials said more than 200 people have been injured.

Some of the children killed were buried when the two-mile wide funnel of wind demolished an elementary school.

US President Barack Obama declared a "major disaster" as crews combed the wreckage of the shattered community, where even residents with long memories of past storms were shocked by the devastation.

In televised remarks from the White House, Obama made special mention of the young victims as he mourned those lost and promised to provide survivors with the help they need to find their footing.

"The people of Moore should know that their country will remain on the ground there for them, beside them as long as it takes for their homes and schools to rebuild," Obama said.

"There are empty spaces where there used to be living rooms and bedrooms and classrooms and in time we're going to need to refill those spaces with love and laughter and community," he added.

The killer system -- packing powerful winds of up to 200 miles per hour (322 kilometers per hour) -- flattened block after block of homes, set off fires, downing power lines and tossed cars.

Stunned weather forecasters described an epic two-mile (three-kilometer) wide mid-afternoon storm, as news helicopter footage showed a dark twister plowing through densely packed suburbs.

"To me, this is bigger than anything I've ever seen. It's absolutely huge. It's horrific," Oklahoma Governor Mary Fallin said.

"It looked like somebody just set off something that just destroyed structures, not blocks, but miles of areas, and major buildings from hospitals to schools to banks to shopping centers, movie theaters."

Local television footage on Monday showed children as young as nine being pulled out of the Plaza Towers Elementary School in Moore, a residential community of 55,000 just south of Oklahoma's state capital.

"I had to hold onto the wall to keep myself safe," one little girl said.

The Moore Medical Center was evacuated after it sustained damage, and state authorities called out the National Guard to help rescue efforts as Obama ordered federal aid to supplement local recovery efforts.

Rescue operations already hindered by the mounds of debris and fallen power lines could be further disrupted by more foul weather.

"I had no idea it was coming," said a horse stable worker, who told how he survived the "unbearably loud" twister by taking cover in one of the stalls.

Monday's tornado followed roughly the same track as a May 1999 twister that killed 44 people, injured hundreds more and destroyed thousands of homes.

Tornadoes frequently stalk Oklahoma's wide open plains but Monday's twister struck a populated urban area. Because of the hard ground, few homes here are built with basements or storm shelters in which residents can take cover.

Oklahoma City lies inside the so-called "Tornado Alley" stretching from South Dakota to central Texas, an area particularly vulnerable to tornadoes, but Moore's residents were shocked at the devastation.

Some 29,000 people remained without power early Tuesday, according to OG&E, the local utility.

The National Weather Service forecast more tornadoes later Tuesday, with parts of Oklahoma, Arkansas, Louisiana and Texas most likely to be affected.