WASHINGTON - The host of one of America's most popular late-night television shows wore a surgical mask to interview Hillary Clinton, taking a dig at the Democratic presidential candidate who was recently diagnosed with pneumonia.

"Welcome, welcome, thank you so much," NBC quoted Jimmy Fallon, presenter of The Tonight Show Starring Jimmy Fallon, as saying as he put on a mask. Clinton laughed at the skit and gave Fallon a high five, after which he jokingly applied hand sanitiser, NBC reported.

The episode will be aired on NBC on Monday night US time (Tuesday Singapore time).

Clinton was diagnosed with pneumonia ahead of a Sept 11 memorial ceremony in New York. Instead of resting, the 68-year-old presidential hopeful continued to work and appeared wobbly at the event, fuelling speculation about her health. As few people knew of her diagnosis, she was criticised for a lack of transparency.

Clinton's campaign then released a letter from her doctor describing her condition as mild and painting an overall picture of good health. After three days of rest, she returned to the campaign trail on Friday.

The popular TV host was critised by some for being too lenient with Republican presidential candidate Donald Trump during an interview last week. It was described as an extended on-air puff piece, filled with the same sort of polite give-and-take a late-night host reserves for an actor on an obligatory promotional tour, the Huffington Post reported.

Throughout the interview, Fallon was predictably non-confrontational and willing to work with whatever Trump gave him, said the report.

He did his very popular impression of Trump's speaking style, ribbed him right and left and concluded his interview with a request. "Can I mess up your hair?" he asked.

Mr Trump grinned and agreed, and Fallon mussed his hair with a vigorous, repeated rub.