The warning came after two anti-tank mines from World War II exploded spontaneously in the ground in the eastern German state of Brandenburg, Der Spiegel reported Thursday.
The first blast occurred Dec. 14 and tore a crater 33 feet deep next to a road 62 miles southeast of Berlin. Another mine exploded Dec. 17, officials said.
Although no one was injured in the blasts, experts say the risk of such blasts from unexploded shells and other munitions left over from World War II is increasing as the trigger mechanisms in the mines deteriorate with age.
Earlier this month, about 45,000 people were evacuated from their homes in Koblenz while experts disarmed a 1.8-ton British bomb found in the Rhine River.
"Germany will have to deal with the munitions problem for a very long time to come," Hans-Jurgen Weise, a former bomb disposal chief for the west of Brandenburg, said.
"Unexploded bombs are becoming more dangerous by the day through material fatigue as a result of aging and through the erosion of safety elements in the trigger mechanisms."