President Barack Obama arrived in Moore, Oklahoma, on Sunday to tour the town flattened by a tornado that killed 24 people.
He assured the residents that the federal government would provide long-term help.
Mr Obama's first stop was Plaza Towers Elementary School, where seven children died and several students and teachers were injured, Reuters reports.
Entire blocks of houses, two schools and a hospital were flattened in 50 minutes on 20 May. Some 377 people were injured.
Piles of boards, brick and cinder blocks that used to be buildings and houses lined the street.
"Obviously the damage here is pretty hard to comprehend," Mr Obama said.
"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.
"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."
