Doha, Qatar: The US ambassador to Qatar said Tuesday she is leaving her post in Doha, in the midst of the worst diplomatic crisis involving America's Gulf allies in years.

"This month, I end my 3 years as US Ambassador to #Qatar. It has been the greatest honor of my life and I'll miss this great country," Dana Shell Smith wrote on Twitter.

Smith did not say why she was stepping down, if she was staying within the diplomatic service or who would replace her.

In Washington, officials said the ambassador had made a personal decision to leave the post earlier this year after serving a normal three-year tour.

"Ambassador Dana Smith's assignment as ambassador comes to an end this month and she will depart Qatar later this month as part of the normal rotation of career diplomats throughout the world," a senior State Department official said.

"Her decision to leave the foreign service was made earlier this year. We wish her the best as she moves on from the Department of State."

Smith's departure comes with Washington sending mixed signals over the Gulf crisis, which saw Saudi Arabia and several of its allies cut ties with Qatar claiming that Doha supported extremist groups.

Qatar strongly rejects the allegations.

US President Donald Trump has signalled his support for the Saudi-led move but other US officials have been more cautious and called for dialogue to end the crisis.

Smith was appointed ambassador to the Gulf emirate by Barack Obama in 2014.

Last month she appeared to express dissatisfaction with political events back home in another message posted on social media.

She took to Twitter in the hours after Trump's dramatic sacking of FBI director James Comey, tweeting: "Increasingly difficult to wake up overseas to news from home, knowing I will spend today explaining our democracy and institutions."

Qatar is home to Al-Udeid, the largest US airbase in the region, which houses around 10,000 troops.