DOHA (AFP) - The US Ambassador to Qatar has expressed her dissatisfaction with political events back home in a decidedly undiplomatic message posted on social media.

Ms Dana Shell Smith, who has been the American envoy to the Gulf emirate since 2014, took to Twitter in the hours after the dramatic sacking of FBI director James Comey by US President Donald Trump.

"Increasingly difficult to wake up overseas to news from home, knowing I will spend today explaining our democracy and institutions," Ms Shell Smith tweeted, in apparent disapproval of the move.

It was a rare public show of exasperation with political events - either in the US or Qatar - by the envoy, who was appointed by former president Barack Obama.

The message was posted on May 9 and has been retweeted more than 2,000 times.

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

Last month, US Defence Secretary James Mattis visited Doha and met Emir Sheikh Tamim bin Hamad Al-Thani.

Trade between the two countries is also growing.

Last year, after Mr Trump's election, it was announced that gas-rich Qatar's sovereign wealth fund wanted to invest US$10 billion (S$14.09 billion) in US infrastructure projects.

There was some angry reaction to Ms Shell Smith's tweet online, with calls for her to be sacked for her apparent criticism of the President.

In a new post on Thursday morning, Shell Smith appeared to try to take the sting out of her earlier message.

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