US president Barack Obama has spoken of his "gratitude" to rescuers who raced to help some of the thousands of people caught up in a deadly storm he described as "one of the most destructive tornadoes in history".
He was speaking as the death toll was revised to 24, including seven children, after the huge tornado ripped through the Oklahoma City suburb of Moore in the US Midwest, destroying scores of homes and a primary school.
Speaking at the White House on Tuesday, president Obama said: "In an instant neighbourhoods were destroyed, dozens of people lost their lives, many more were injured and among the victims were young children trying to take shelter in the safest place they knew - their school.
"Our prayers are with the people of Oklahoma today. Our gratitude is with the teachers who gave their all to shield the children, with the neighbours, the first responders and emergency personnel who raced to help as soon as the tornado passed, and with all of those who, as darkness fell, searched for survivors through the night."
The president declared Oklahoma a major disaster area and ordered federal aid to supplement state and local efforts in Moore after the deadliest US tornado since 161 people were killed in Missouri two years ago.
"The people of Moore should know that their country will remain on the ground, there for them, beside them, as long as it takes," he said.
His words followed that of the Queen who earlier said she was "deeply saddened" by the loss of life and devastation caused by the tornado.
President Obama added: "As a nation our full focus right now is on the urgent work of rescue and the hard work of recovery and rebuilding that lies ahead.
"The people of Moore should know that their country will remain on the ground, there for them, beside them, for as long as it takes for their homes and schools to rebuild, businesses and hospitals to reopen, parents to console, first responders to comfort and of course frightened children who will need our continued love and attention."
The president said the people of Oklahoma have all the resources they need at their disposal, adding that the full extent of damage would not be known for some time.
The death count from the disaster was previously thought to be as many as 90, but the state medical examiner's office revised this as it was thought that some victims were counted twice in the early chaos of the storm.
Rescue teams are continuing to search the rubble after the deadly tornado hit yesterday afternoon, local time.
Emergency services are still searching the rubble of the devastated Plaza Towers Elementary School, which took a direct hit in the powerful storm, its roof ripped off and walls flattened.
A water pipe is believed to have burst at the school and a number of the children could have drowned after becoming trapped in the debris.
Oklahoma's Lt Gov Todd Lamb told the BBC: "The school was flattened. The walls were pancaked in. There have been some bodies recovered from that school and it's absolutely horrific and devastating."
Worst hit was the town of Moore, street signs disappeared under flying detritus, lights went out, and houses, schools and a hospital were completely obliterated.
In a message of condolence to president Obama, the Queen said: "I was deeply saddened to hear of the loss of life and devastation caused by yesterday's tornado in Oklahoma.
"Prince Philip joins me in offering our heartfelt condolences to the victims and their families at this difficult time.
"Our deepest sympathies go out to all those whose lives have been affected, as well as the American people."
The message was signed Elizabeth R.
Save the Children said it has launched an emergency response to the disaster.
The aid agency, which has led and supported resilience-building programmes in Oklahoma over the past five years, is preparing to distribute essential supplies to families and communities affected by the tornado.
These include infant and toddler hygiene materials and kits to create safe play areas.
Save the Children has previously helped children affected by major emergencies in the US, including hurricane Katrina and the 2011 tornadoes that devastated areas of Tuscaloosa, Alabama, and Joplin, Missouri.