US Democratic presidential nominee Hillary Clinton, who is recuperating from pneumonia, plans to resume campaigning on Thursday, her spokesman said.

Clinton "has spent the day catching up on reading briefings, making calls" and watching on television President Barrack Obama's speech at a campaign appearance for the former secretary of state in Philadelphia, spokesman Nick Merrill said on Tuesday.

"We will resume campaign travel on Thursday, more details to come," he added.

Hours after Clinton, 68, almost collapsed on Sunday at a New York memorial ceremony for the Sept. 11, 2001, attacks, her doctor said she had been diagnosed with pneumonia two days earlier.

Clinton said on Monday that she would rest a few days before resuming her campaign for the Nov. 8 election against Republican Donald Trump.

Clinton said she had ignored doctor's order to rest and had not revealed her diagnosis earlier because "I just didn't think it was going to be that big a deal."