An experienced Russian cosmonaut scheduled to fly to the International Space Station in 2015 has resigned for reasons that are unclear, officials say.

Yury Lonchakov will formally leave his job Sept. 14, Irina Rogova of the Gagarin Cosmonaut Training Center told RIA Novosti.

Russian media quoted the training center's head, Sergei Krikalev, as saying Lonchakov "found a more interesting job," but he did not elaborate.

No replacement has been announced announced for Lonchakov, who was set to fly as the commander of Expedition 44 to the ISS in May 2015 with Russian cosmonaut Mikhail Korniyenko and NASA astronaut Scott Kelly.

Lonchakov, 48, had been a cosmonaut since 1998 and made three space trips between 2001 and 2009 spending a total of 200 days in orbit, and made two spacewalks with a combined duration of more than 10 hours.

The Gagarin Cosmonaut Training Center has had organizational problems since 2009, when it was transferred from the Defense Ministry to the civilian Federal Space Agency.

One Russian cosmonaut told a newspaper last year the prolonged reorganization had affected cosmonauts' income and career prospects, breeding discontent among personnel.