The body of the first Russian serviceman confirmed dead in four weeks of air strikes in Syria was delivered on Tuesday to his parents, who said they were not convinced by the military's account that their 19-year-old son had hanged himself.

In an interview with Reuters at their home in southern Russia before they received the body of their son Vadim, Alexander and Svetlana Kostenko said their son had sounded cheerful over the phone as recently as Saturday, the day he died while working at an air base on the Syrian coast.

"I will never believe this version (suicide)," said Svetlana, who was wearing a black head scarf. "We spoke every day by phone for half an hour. (On Saturday) he was cheerful, happy, and he laughed," she said.