The US embassy in Banjul was closed for all but essential services today after Gambian authorities removed its police protection.
It was not immediately clear why President Yahya Jammeh's government had taken this step, but Washington has criticized its rights record.
A State Department official in Washington told AFP the United States had "registered concern" with Gambia over the withdrawal of the officers.
"We will continue to closely monitor and assess the situation as events unfold," he said.
"The US embassy sent a security message to citizens to alert them to the embassy closure."
An AFP reporter in Banjul who visited the US mission confirmed that the Police Intervention Unit officers normally deployed there were gone.
The spat comes amid controversy over Gambia's arrest and prosecution of a US citizen who was detained while visiting relatives in the country.
Fanta Darboe Jawara, a 45-year-old naturalized US citizen from Frederick, Maryland, was held on April 16 after an opposition protest.
Her family told US media she was not involved in the protest and was simply waiting for a taxi when she was arrested and beaten by police.
But the president of Gambia's opposition UDP party told AFP that Jawara had been detained at the home of human rights lawyer Ousainou Darboe.
Darboe's house is a short distance from the US embassy, UDP president Dembo Bojang said.
US officials say they are aware that Jawara has family connections with the Gambian opposition, but have not commented in detail on the case.
A State Department official would not speculate on whether the police withdrawal was connected to controversy over Jawara's arrest.
"We will continue to monitor her case and provide all possible consular assistance," he said.
Jammeh seized power in a bloodless coup in 1994 and is regularly accused of a catalogue of rights abuses.
He recently told UN chief Ban Ki-moon and Amnesty International to "go to hell" for urging an inquiry into the death of a protestor in custody.
Opposition activists have borne the brunt of a crackdown on basic rights ahead of polls expected to re-elect Jammeh for a fifth term.
