By Caren Bohan

JOPLIN, Missouri (Reuters) - President Barack Obama said Sunday that a tornado that killed at least 139 people in this small town a week ago was a national tragedy that will require a national response.

"We're going to be here long after the cameras leave," he said, referring to federal support for rebuilding efforts that he predicted would be "a tough, long slog."

Obama, who returned Saturday night from a six-day trip to Europe, toured a disaster scene where crushed cars, piles of wood, clothing and a broken dishwasher lay helter-skelter amid the rubble on lots where houses once stood.

Tree trunks, their branches shorn, were white where the bark was stripped nearby.

The May 22 tornado was the deadliest single twister in the United States since 1947. It killed at least 139 people and injured more than 900. Scores are still unaccounted for.

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 Missouri Governor Jay Nixon, a Democrat, and U.S. Senator Claire McCaskill, a Democrat of Missouri.

Obama, who was to speak at a memorial service for victims in an auditorium at Missouri Southern State University, only a few miles (km) from the worst damage, vowed to cut through any federal red tape to help with rebuilding.

"This is a national tragedy," he said.

(Reporting by Caren Bohan; Editing by Sandra Maler)