The authorities in Zimbabwe have rejected an offer by the FBI to help investigate three Americans arrested last week with a truckload of guns at Harare airport.

A US spokeswoman said an FBI agent had now left Zimbabwe after the authorities made it clear that his help was not wanted.

The three suspects are alleged to have smuggled the arms from the Democratic Republic of Congo.

They have been charged with possession of arms, terrorism, espionage and acts of sabotage.

A senior Zimbabwean government official told the BBC that, as the suspects were American citizens, the FBI would not be neutral in its investigations.

He said there were also fears that they may have been operating on behalf of the United States government -- which Zimbabwe suspects of supporting attempts to depose the President of the Democratic Republic of Congo, Laurent Kabila.