WASHINGTON (Reuters) - The senior U.S. diplomat for Asia, Susan Thornton, will retire at the end of July, the State Department said on 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.

“We are grateful for her service of over two and a half decades to the Department of State, including numerous challenging assignments around the world,” Nauert added.

Nauert did not say when Pompeo planned to announce a replacement, but said: “He is moving forward with efforts to nominate candidates for leadership roles across the department, including for this key position.”

Thornton’s retirement comes as President Donald Trump’s administration prepares for negotiations to ensure Pyongyang abandons its nuclear weapons, following a summit between Trump and North Korean leader Kim Jong Un in Singapore.

Pompeo, who is charged by Trump with leading follow-on negotiations, said the United States hoped to achieve “major disarmament” by North Korea within the next 2-1/2 years.

Thornton’s departure also comes amid trade tensions between Washington and Beijing at a time when Trump is pressing China for cooperation on North Korea.

Thornton could not be immediately reached for comment on Saturday.

Separately, the State Department said the U.S. ambassador to Estonia, Jim Melville, had decided to step down from the foreign service after 33 years.

Media reports said Melville, who has been ambassador to Estonia since 2015, was due to retire but told friends in a private Facebook posting that he was resigning amid comments by Trump about U.S. allies in Europe.

“Yesterday, the United States’ Ambassador to Estonia, Jim Melville, announced his intent to retire from the Foreign Service effective July 29,” a State Department official said.

Melville could not be reached to confirm the reports that he had stepped down over Trump’s remarks.

Tensions between the United States and Europe have escalated since Trump announced in May he was pulling out of the 2015 Iran nuclear deal and, separately, said he would impose tariffs on European steel and aluminum exports.