"The liberation of the last (Daesh)-held territory is a historic milestone that would not have been possible without their commitment, professionalism and courage," she said in a statement on Saturday.
Britain is part of the US-led coalition claimed to be fighting Daesh terrorists in Syria, Presstv Reported.
American and British leaders have tried to take credit of major victories achieved in Syria by Syrian forces.
On Friday, US President Donald Trump said Daesh no longer held any territory in Syria. White House press secretary Sarah Huckabee Sanders told reporters aboard Air Force One that the territorial Daesh-held had been “100 percent eliminated” from the Middle Eastern country.
In December, the US president said he would withdraw all 2,000 troops from Syria. However, the Wall Street Journal said recently that Washington planned to keep about 1,000 troops in Syria.
US Joint Chiefs Chairman General Joseph Dunford later rejected the report as “factually incorrect,” stressing that there was no change to the planned 400 troops which the White House wanted to keep in Syria.