Italian Ambassador Prio Mariani sustained a minor head injury, and U.S. Embassy officials in Colombo said U.S. Ambassador Robert Blake was uninjured.
The men were in chartered helicopters along with several Sri Lankan government officials and were attacked as they landed at a stadium in the east-central coastal city of Batticaloa, the Colombopage news service reported.
The group was scheduled to tour the area and talk on the Tigers, who are fighting for independence from Sri Lanka.
A military spokesman said the mortar attack injured eight police and soldiers and a schoolboy.
The helicopters were not damaged and evacuated the officials to Colombo, the report said.