 -- A Russian military transport plane has crashed in the south of the country during a training flight, killing one crew member and injuring four others, authorities said.

Russia’s defense ministry said the Antonov AN-26 transport crashed as it came into land after a training flight at Balashov airbase in the Saratov region and caught fire after it hit the ground.

The surviving crew were being treated at a nearby military hospital, some for serious injuries, Russia’s Investigative Committee, the country’s equivalent of the FBI, said in a statement.

There had been four pilots and a cadet aboard the plane, the statement said.

The Investigative Committee said it appeared the crash may have been caused by the failure of one of the plane’s engines but that it was still investigating. The Committee said it had opened a criminal investigation into the incident, examining whether flight rules and preparations for it had been properly followed.

The AN-26 is a Soviet-era transport plane, still widely used by the Russian military.

In recent years, Russia’s air force has suffered a series of deadly crashes, mostly caused by technical failure. Last summer, Russia grounded around 300 of its Su-27 fighter jets after a fatal crash. In 2015, it also grounded its fleet of long-range Tupolev Tu-95 bombers that serve as part of the country’s nuclear deterrent after one of the Cold War-era planes caught fire.

Aviation experts have blamed the crashes in part on Russia’s ageing fleet of military aircraft largely constructed under the Soviet Union, but note the uptick in accidents has also coincided with increased activity by the Russian air force in the past two years.

Since invading Ukraine in early 2014, Russia’s military has been staging frequent snap exercises and is also running an intense air campaign in Syria, putting greater strain on its older aircraft.