The Russian Defense Ministry says one of its jets has crashed during a training flight over the Sea of Azov and that two pilots are missing.
The pilots ejected from the L-39 aircraft before it went down in the water off the southwestern Krasnodar region on October 18, the ministry said.
A search is under way for the personnel.
Russian news agency quoted a source as saying the pilots are believed to have survived the incident but have not yet been recovered by search crews.
"The plane was performing its flight without ammunition. Preliminary findings indicate that the crash could have been caused by mechanical failure," the Defense Ministry said.
Russia generally uses the Czech-made L-39 training craft, developed in the 1960s, at its flight schools, as do many other countries.