"The cameras may leave. The spotlight may shift," he told a memorial service on Sunday for the 139 known victims of the May 22 twister. But the federal government "will be with you every step of the way until Joplin is restored and this community is back on its feet," Mr. Obama said to a standing ovation from survivors.

Before the service in an auditorium at Missouri Southern State University, Obama rolled up his sleeves and toured a disaster scene where crushed cars, piles of wood, clothing and a broken dishwasher lay helter-skelter amid the debris on lots where houses once stood.

The president, who returned on Saturday night from a six-day trip to Europe, vowed to cut through any federal red tape to help with rebuilding that he predicted would be "a tough, long slog."

"This is just not your tragedy," he said after meeting survivors. "This is a national tragedy and that means there's going to be a national response."

The tornado was the deadliest single twister in the United States since 1947. In addition to the at least 139 people killed and more than 900 injured, scores are still unaccounted for a week later.

The tornado churned through a stretch nearly a mile wide, damaging about 8,000 buildings in Joplin, a city of 50,000 in southwestern Missouri.