No US military official could be reached for comment.
"There was a threat from alleged terrorists to harm the American troops," a source in the military’s Southern Command headquarters based here said.
Barangay leaders advised residents that the scheduled free medical and dental checkup had been canceled, but they were not told the reason why.
A resident said they were advised that the medical mission had been reset to today, but that American soldiers would not be around.
The medical mission would have been held at the Arena Blanco Public School. Filipino and American military officials were putting in place security measures when US troops were reportedly ordered to immediately pack up.
The school is in Barangay Arena Blanco, some eight kilometers east of Zamboanga City.
US troops have been conducting joint medical missions with the local military and non-government organizations to serve hundreds of villagers since their arrival last year in Zamboanga City and Basilan.
About 300 American troops are still in Zamboanga City providing anti-terror training to four Army Light Infantry Battalions. This is expected to end by December.
US President George W. Bush, in his eight-hour state visit to the country last Oct. 18, vowed to continue helping the Philippines fight terrorism.