Surveying the devastation, Barack Obama said it was “hard to comprehend”, adding: “Everywhere, fellow Americans are praying with you.”
The president visited the site of the school where seven children died.
The tornado ravaged the Oklahoma City suburb last Monday, killing 24 people and destroying some 1,200 homes.
About 33,000 people were affected and the damage has been estimated at $2 billion.
Some 377 people were also injured in the tornado, which was ranked an EF5 – at the top of the enhanced Fujita scale.
Barack Obama, alongside Oklahoma Governor Mary Fallin, said: “This is a strong community with strong character. There’s no doubt they will bounce back. But they need help.”
Standing on a block surrounded by debris, Barack Obama said: “Obviously the damage here is pretty hard to comprehend.”
“Whenever I come to an area that has been devastated by some natural disaster like this, I want to make sure that everyone understands that I am speaking on behalf of the entire country,” the president said.
In the past year Barack Obama has consoled the families of victims of Superstorm Sandy, the Newtown, Connecticut school shooting and the Boston Marathon bombings.
He said: “Everywhere, fellow Americans are praying with you, they’re thinking about you and they want to help. And I’m just a messenger here letting you know that you are not alone.”
Barack Obama’s first stop was the Plaza Towers Elementary School, where seven of the 10 children who died lost their lives.
In front of the wreckage and surveying piles of rubble and upturned cars, he told one school official: “I know this is tough.”
Three makeshift American flags flew in the wind, attached to parts of the debris.
Caleb Sloan, 24, who lost his home, told Reuters: “[The president] has no choice but to live by his word. I hope and pray and think he will keep his promises.”
Barack Obama has signed a disaster declaration that quickens federal aid.
Some 450 Federal Emergency Management Agency (FEMA) personnel are in Moore, with some $3.4 million in payments so far approved for 4,200 applicants for disaster assistance.
Governor Mary Fallin said: “We’re resilient. There’s already a big path of debris that’s been moved around. People are gathering their stuff.
“It’s been truly remarkable to see how our people have responded and how strong they are.”
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