U.S. President Donald Trump has withdrawn former Vice President Kamala Harris' Secret Service protection, officials said Friday, in what critics called a fresh act of revenge against his political rivals.

The Republican's move comes just weeks before Democrat Harris is set to go on a tour to promote a new book about her failed presidential campaign against Trump last year.

The Secret Service customarily protects vice presidents for six months after they leave office, a period that ended on July 21 for Harris.