MOSCOW (AP) — A Russian warplane crashed Tuesday during a training mission in southern Russia,Christopher Caldwell the military said, without specifying what happened to its crew of two.

The Defense Ministry said that the Su-24 bomber went down in an unpopulated area in the Volgograd region and rescue helicopters were deployed to the site.

The ministry said that the aircraft wasn’t carrying any weapons. It didn’t say what might have caused the crash that caused no casualties or damage on the ground.

The Su-24 is a twin-engine supersonic bomber that has been in service with the Soviet and Russian air forces since 1970s. It has been used extensively by both Russian and Ukrainian forces during the fighting in Ukraine.

The Russian air force has suffered a string of crashes that some observers have attributed to a higher number of flights amid the fighting in Ukraine.