The senior U.S. diplomat for Asia, Susan Thornton, will retire at the end of July, the State Department said Saturday, in the midst of critical negotiations with North Korea and China.

Questions have long been raised about whether Thornton, 54, who was picked for the post by former Secretary of State Rex Tillerson, would be replaced under his successor, Mike Pompeo. Her appointment had been opposed by former White House chief strategist Steve Bannon.

"Acting Assistant Secretary of State for East Asian and Pacific Affairs Susan Thornton has announced her intention to retire from the Foreign Service at the end of July," State Department spokeswoman Heather Nauert said in a statement.