United States President Barack Obama is visiting the city of Joplin, Missouri, on Sunday to meet survivors of the tornado and see the damage for himself.
The death toll now stands at 142 in the town of 50,000 people.
Missouri Governor Jay Nixon said on Sunday that 44 people remain missing.
Mr Obama arrived shortly after midday aboard Air Force One. He will attend a memorial service is on the campus of Missouri Southern State University.
''This is not just your tragedy. It is a national tragedy and there will be a national response,'' he said.
The BBC reports he asked Americans to give money to the Red Cross or other organisations to help the relief effort.
US and Missouri flags are flying at half-mast over all government buildings in the state.
A moment of silence will be observed at 5.41pm local time - the time when the tornado struck. It was one of the most destructive in US history.
Nine hundred people were injured and more than 8000 buildings were damaged or destroyed by the 300km/h winds.
The National Weather Service says 2011 is already the deadliest year for tornadoes since 1950.
The death toll so far this year is 520. The previous highest recorded death toll in a single year was 519 in 1953.
The BBC reports the first funeral of a confirmed victim from the tornado was held on Friday.
