Democratic presidential nominee Barack Obama threw himself back onto the campaign trail on Saturday, saying he was deeply grateful for an outpouring of support over his gravely ill grandmother.

"Some of you know that I have been off the trail for a day," Obama said, at a rally in Reno, Nevada, less than nine hours after flying back to the US mainland from his compassionate leave in his native Hawaii.

"I just want to personally say how grateful I am that so many people sent out their thoughts and prayers sent my grandmother flowers, get well cards," he said.

"I just want you to know it meant the world to her, it means the world to me," Obama said. "Thank you everybody for being so gracious."

"It means a lot, it means a lot, it means a lot."

Saturday's comments were the first time that Obama had talked about the health of his grandmother Madelyn Dunham, who turns 86 on Sunday, at a campaign event.

He said in an interview with ABC broadcast on Friday that he was not sure that Dunham, who raised him during his childhood years in Hawaii, would live to see election day on 4 November.

AFP