US president Donald Trump has said his wife, Melania, had a "big operation" to treat a kidney condition that lasted close to four hours, and cannot accompany him to the G7 meeting in Canada under the orders of her doctor.

Mr Trump added that the US first lady is "doing great".

He said Mrs Trump had wanted to join him at an annual summit of leading industrialised nations, as she did last year, but that she "can't fly for one month".

He said: "First lady's great. Right there," pointing up to the White House from the driveway. "And she wanted to go. Can't fly for one month. The doctors say. She had a big operation. That was a close to a four-hour operation. And she's doing great. Right there."

Mr Trump added: "And you know what? She is a great first lady."

The US leader's comments only added to the mystery surrounding his wife's recent trip to hospital and near four-week absence from the public eye.

The first lady's office announced on May 14 that she had had an embolisation procedure to treat a kidney condition described as benign. She spent five nights in a military hospital before returning to the White House on May 19 to continue her recuperation.

Her office has declined to provide additional details about her condition, citing privacy rights.

Mrs Trump's absence, beginning soon after she announced her initiative to teach kindness to children, sparked several theories about where she was and what might have happened to her, including that she had moved back to the family's penthouse at Trump Tower in New York.

She ended the speculation this week by making her first public appearance in nearly a month, accompanying Mr Trump to a hurricane preparedness briefing at Federal Emergency Management Agency headquarters in Washington on Wednesday.