Hillary Clinton formally stepped down as US secretary of state on Friday with a letter to President Barack Obama and an emotional farewell address to the department she has run for four years.   "I leave thinking of the nearly 70,000 people that I was honored to serve and lead as part of a huge extended family. And I hope that you will continue to make yourselves, make me, and make our country proud," the 65-year-old former first lady said.

A day after resigning from the Senate, John Kerry was sworn-in as Clinton's successor in a private ceremony late Friday.

"So it's been quite a challenging week saying goodbye to so many people and knowing that I will not have the opportunity to continue being part of this amazing team," Clinton told state department staff.

Her tenure ended on a turbulent note, as two people died in a suicide bombing outside the US Embassy in Turkey.

Clinton has said that she plans to write a memoir and remain active in efforts to expand the role of women in politics.