WASHINGTON: Hillary Rodham Clinton has formally resigned as the 67th US secretary of state, capping a four-year tenure in the office that saw her shatter previous records for countries visited.    

In a letter sent to President Barack Obama on Friday shortly before she was to leave the State Department for the last time in her official capacity, Clinton thanked her former foe for the 2008 Democratic presidential nomination for the opportunity to serve in his administration.

Clinton said it had been an honor to be part of his Cabinet and that she remained convinced of the ''strength and staying power'' of American global leadership.

Her resignation will be effective on the swearing-in of her successor, John Kerry, who will take the oath of office in a private ceremony later Friday.