WASHINGTON — White House Chief of Staff John Kelly's long-expected departure will likely occur in the days ahead, four sources tell NBC News.

His tenure has been marred by controversy and conflict since he became chief of staff in the summer of last year, amid reports of disagreements with the president and West Wing staff. Recent clashes with first lady Melania Trump made his already tenuous position even more so, NBC News reported last month.

Kelly was tapped by Trump as chief of staff in July 2017 after his predecessor, Reince Priebus, was ousted following six months on the job.

Kelly previously served as Trump's secretary of homeland security and is a retired four-star Marine general who served in the military for more than four decades, making him one of the nation's longest-serving commanders.

Kelly's time in the West Wing has also been dogged by speculation that he is frustrated and exhausted by his job and the president's mercurial temperament — and that the president has at times been angry and vented his own frustrations about Kelly.

Among the leading candidates to replace him is Vice President Mike Pence’s chief of staff, Nick Ayers.

Carol E. Lee, Kristen Welker, and Hallie Jackson reported from Washington, and Dartunorro Clark reported from New York.