|Barack Obama and Missouri Governor Jay Nixon toured the scene of the tornado disaster in Joplin [Reuters]
US president Barack Obama has pledged government help to assist the victims of the deadliest tornado to hit the country in 65 years, following a memorial service for those killed in the Missouri disaster.
Obama toured the tornado site in the city of Joplin on Sunday, and offered his condolences at a service honouring the 139 known victims of the May 22 twister.
He called the incident a national tragedy that will require a national response.
"We're going to be here long after the cameras leave," Obama said, referring to federal support for rebuilding efforts that he predicted would be "a tough, long slog".
He said that it was impossible to know when or why such devastation strikes, but praised neighbours for helping each other at great risk to themselves, saying "there are heroes around us all the time".
Obama, who returned to the US on Saturday night from a six-day trip to Europe, toured a disaster scene where crushed cars, piles of wood and debris lay strewn amid the rubble on lots where houses once stood.
The May 22 tornado was the deadliest single twister in the United States since 1947, killing at least 139 people and injured more than 900. At least 40 more remain unaccounted for.
"This is not just your tragedy. This is a national tragedy, and that means there will be a national response," Obama said.
The tornado slammed into Joplin, a city of 50,000 in southwest Missouri, and left a swath of destruction nearly a mile wide, damaging about 8,000 buildings.
Standing amid the wreckage, Obama told reporters after meeting survivors that he had heard some "harrowing stories" but also some "miraculous ones".
He said he had spoken with an 85-year-old man who had just taken a chicken pot pie out of the oven when the storm was approaching.
"He went into the closet and came out without a scratch," Obama said.
The president got a big-picture view of the damage from his plane before landing. He was accompanied on the tour by Jay Nixon, a Missouri Governor, and Claire McCaskill, US Senator from Missouri.