The State Department late Tuesday said "this historic event reflects Somalia's progress in recent years and is another step forward in formalizing U.S. diplomatic engagement in Mogadishu."
The U.S. closed its Somalia embassy in January 1991 as the country collapsed into civil war, with the military airlifting out the ambassador and others.
The U.S. formally recognized Somalia's new federal government in 2013 but has based its diplomatic mission in the capital of neighboring Kenya.
Former Secretary of State John Kerry during a surprise 2015 visit to Somalia said the U.S. would begin the process of re-establishing a diplomatic presence.
New Ambassador Donald Yamamoto arrived at the U.S. mission last month.
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