Veteran Democrat John Conyers announced Tuesday he is retiring from the US House of Representatives after more than five decades in office, following a series of sexual harassment accusations by former staffers.

"I am retiring today," Conyers, 88, told a Detroit radio station, as he became the first to step down among four sitting US lawmakers facing sexual misconduct allegations.

Speaking from an unnamed hospital where he reportedly was being treated for stress-related complications, the longest-serving member of Congress maintained that the allegations against him by several women "are not true."