First lady Melania Trump is expected to remain in hospital for the rest of the week after a "long-planned" procedure to treat a benign kidney condition, White House officials have said.

Mrs Trump, 48, had an embolisation procedure on Monday at Walter Reed National Military Medical Centre outside Washington, her staff said.

President Donald Trump visited her later on Monday and tweeted that the procedure was "successful" and that his wife was in "good spirits".

The White House did not offer any additional details on Mrs Trump's condition, though vice president Mike Pence described the procedure as "long planned".

"Melania is already on the mend," he added.

She was last seen in public on Wednesday at a White House event where she and the president honoured military mothers and spouses for Mother's Day.

Experts said the most likely explanation for the procedure is a kind of non-cancerous kidney tumour called an angiomyolipoma.

They are not common but tend to occur in middle-aged women and can cause problematic bleeding if they become large enough, said Dr Keith Kowalczyk of MedStar Georgetown University Hospital.

"The treatment of choice" is to cut off the blood supply so the growth shrinks, added Dr Lambros Stamatakis of MedStar Washington Hospital Centre.

Doctors do that with an embolisation, meaning a catheter is snaked into the blood vessels of the kidney to find the right one and block it.

The Slovenia-born former model married Mr Trump in 2005. They have a 12-year-old son named Barron.

Mrs Trump, who has been gradually raising her profile as first lady, recently hosted her first state dinner and launched a public awareness campaign to help children.