US ambassador to Qatar is leaving in the middle of a Persian Gulf crisis

• Dana Shell Smith on Tuesday said on Twitter she will end her three-year stint as U.S. ambassador to Qatar.
• She is leaving her post shortly after a Saudi-led coalition cut diplomatic ties with Qatar.
• Shell Smith has recently appeared to criticize President Donald Trump in tweets.

The U.S. ambassador to Qatar announced she is leaving her post, just one week after several neighboring countries cut diplomatic ties with the small Gulf monarchy.

Dana Shell Smith on Tuesday said on Twitter that this month she will end her three-year stint as America's top diplomatic envoy to Qatar, which hosts the largest U.S. military presence in the Middle East.

@AmbDana: This month, I end my 3 years as U.S. Ambassador to #Qatar. It has been the greatest honor of my life and I'll miss this great country.

@AmbDana: I will be announcing my personal Twitter account soon. Keep an eye out!

The State Department told CNBC that the ambassador's assignment had come to an end and that she decided to retire from the Foreign Service earlier this year. A department spokesperson referred a question about Shell Smith's interim and long-term replacement to the White House.

Shell Smith did not immediately return a request for comment. It's not unusual for diplomats to move on from their posts when the presidency swings from one political party to another.

Critic of President Trump

Shell Smith has recently written tweets that appeared critical of Donald Trump.

Following the president's abrupt firing of FBI Director James Comey, Shell Smith wrote that it can be hard to explain U.S. events to foreign listeners.

@AmbDana: Increasingly difficult to wake up overseas to news from home, knowing I will spend today explaining our democracy and institutions.

@AmbDana: Diplomats explain & defend our political system.Can be tough when partisan acrimony so high, but there is still no greater country. #USA

Just hours after Trump struck out at London Mayor Sadiq Khan following a deadly terror attack on the city, Shell Smith retweeted a message from Los Angeles Mayor Eric Garcetti praising Khan.

@ericgarcetti: To my brother mayor @SadiqKhan LA is with you and all of the people of London, where I once lived. Thank you for all you are doing. EG

Trump last week appeared to back the anti-Qatar coalition — and even take partial credit for isolating Doha — but Shell Smith defended Qatari efforts to rein in terror financing following the diplomatic breakdown. She also retweeted official State Department messages calling for a de-escalation of tensions.

@AmbDana: Seems a good time to RT this one.

Saudi Arabia, the United Arab Emirates, Egypt and other nations cut ties and severed transportation links with Qatar last week, accusing the nation of supporting terrorism and allowing financing for extremist groups to flourish within its borders.

Shell Smith's departure adds to a long list of vacancies at the State Department, which has raised concerns about the country's ability to manage overseas crises.