US President Barack Obama has described last week's devastating tornado in the city of Joplin, Missouri, where 142 people are now known to have died, as a "national tragedy".
Visiting the city, Mr Obama said the disaster required a national response.
He took part in a memorial service and observed a minute of silence at 1741 local time (2241 GMT) - exactly a week after the tornado struck.
The tornado was one of the most destructive in US history.
With winds of 200mph (322km/h), it carved a swathe of destruction through the city, injuring more than 900 people.
On Saturday, a list of 156 missing fell to 105 after more were accounted for.
Mr Obama drove through the hardest-hit areas with Missouri Governor Jay Nixon, stopping to talk to residents.
He asked Americans to give money to the Red Cross or other organisations to help the relief effort. He also had a message for the city's inhabitants.
"This is not just your tragedy, it is a national tragedy and there will be a national response," he said.
Later he attended a community memorial service at the Taylor Performing Arts Center on the campus of Missouri Southern State University.
In a speech at the service, he added, to enthusiastic applause: "There's no doubt in my mind that Joplin will rebuild, and as president I can promise you, we will be with you every step of the way."
He said: "You've demonstrated a simple truth... that amid heartbreak and tragedy no-one is a stranger. Everybody is a brother. Everybody is a sister. We can all love one another."
The BBC's Paul Adams in Washington says this is the third time this year - after the Tuscaloosa tornadoes and Mississippi floods - that Mr Obama has felt the need to be at the scene of natural disasters to reassure the population they will not have to fend for themselves.
At 1741 Central time, hundreds stood in Joplin's Cunningham Park to observe a moment of silence for the victims.
Governor Nixon had declared Sunday an official State Day of Prayer and Remembrance.
"I invite all Missourians to pause and remember their neighbours and draw upon the resources of their faith in support of their fellow Missourians," the governor said in a statement.
US and Missouri flags are flying at half-mast over all government buildings in the state throughout the day.
On Saturday, city manager Mark Rohr said Mr Obama's visit would be an opportunity to convey the scope of the damage to the federal government.
Mr Rohr also said the death toll now stood at 142 - an increase of three from the previous total.
Among the newly confirmed victims was teenager Will Norton, who was sucked from his father's car as they drove home from his high school graduation.
More than 600 volunteers and 50 dog teams are still scouring the shattered remains of homes and offices for survivors or victims.
"We're going to be in a search and rescue mode until we remove the last piece of debris," Mr Rohr said.
Joplin police say they have made 17 arrests for looting.
The US National Weather Service says that 2011 is already the deadliest year for tornadoes since 1950, when precise figures were first kept.
The death toll so far this year stands at 520. The previous highest recorded death toll in a single year was 519 in 1953.
The first funeral of a confirmed victim from the Joplin tornado was held on Friday.
Hundreds of mourners gathered at a church in Galena, just over the Kansas border, for the funeral of Adam Dewayne Darnaby, 27.
So far at least 19 bodies have been released to families, but many are yet to be formally identified.
Officials say that, wherever possible, they prefer to base identifications on DNA, medical records and also distinguishing features such as tattoos and piercings.
However, some families of victims say the delays are adding to their distress.