WASHINGTON — The State Department's third-highest-ranking diplomat, Undersecretary of State for Political Affairs Nicholas Burns, is resigning, a senior U.S. official said Friday.

Secretary of State Condoleezza Rice planned to appear at a ceremony with Burns at 9:45 a.m. Friday morning to announce his departure, as well as a possible replacement, the official said, speaking on condition of anonymity because the formal announcement had not been made.

Burns, a career diplomat, has led the Bush administration's diplomatic efforts to impose new U.N. sanctions on Iran and has tried to negotiate a controversial nuclear deal with India.