French authorities are investigating the theft of roughly 200 detonators plus grenades and plastic explosives from a military site in southeastern France.

The gendarmerie police force says the thefts at the Miramas site west of Marseille appeared to have occurred overnight Sunday to Monday.

Top French security and defense officials were meeting Tuesday about the theft. A gendarmerie official said the thief or thieves appeared to have cut through a fence to enter the high-security site. He was not authorized to speak publicly because the operation is ongoing.

Mayor Frederic Vigouroux of Miramas told the BFM television network the theft appeared to be "very professional."

France has been on its highest level of alert for terrorism following deadly attacks in January and last month.