US President Donald Trump chief of staff Susie Wiles has been diagnosed with breast cancer but will continue to work during treatment, Trump said Monday.

The first woman to ever work as White House chief of staff, 68-year-old Wiles has been widely credited with driving Trump's second presidency forward behind the scenes.

"Susie Wiles is an incredible Chief of Staff, a great person, and one of the strongest people I know but, unfortunately, she has been diagnosed with early stage breast cancer, and has decided to take on this challenge, IMMEDIATELY, as opposed to waiting," Trump posted on his Truth Social network.

"She has a fantastic medical team, and her prognosis is excellent!"  

Trump added that "during the treatment period, she will be spending virtually full time at the White House, which makes me, as President, very happy!"

Trump is facing a host of challenges ranging from the Iran war to poor approval ratings for his Republican party ahead of crucial midterm elections in November.

Trump has previously dubbed Wiles -- who started working for him in Florida during his 2016 run for the presidency -- "The Ice Maiden" and "the most powerful woman in the world."

She keeps a low profile in public, but in private Wiles is a steely gatekeeper whose main mission has been to avoid any repeat of the chaos and infighting that marked Trump's first spell in the White House.