Thieves have stolen radio equipment from a Russian military plane known as the "doomsday aircraft" for its role in the country's nuclear arsenal, state media report.
The reports say unknown thieves broke into the Ilyushin Il-80 plane at an airfield in the southern Rostov region.
They reportedly opened the cargo hatch and stole 39 pieces of radio equipment.
The local government's transport ministry said an investigation into the break-in was under way.
Military experts say the aircraft is one of four Il-80s designed to be used as airborne command posts for Russian officials, including the president, in the event of a nuclear conflict.
In a report on Monday, state news agency Interfax said the aircraft had been undergoing scheduled repairs at an airfield in the port city of Taganrog since the beginning of 2019.
Officials from the Taganrog Aviation Scientific and Technical Complex reported the theft to local police on 4 December, the agency said.
Citing an unnamed Russian police source, the agency said a cargo-hatch breach was discovered during an inspection of the aircraft. It added that authorities were considering opening a criminal investigation.
All equipment had been intact at the last inspection on 26 November, said the Ren-TV broadcaster, which first reported the break-in. Investigators took fingerprints and shoeprints from inside the aircraft, the broadcaster added.
The aircraft is one of four Il-80s currently in service with the Russian Air Force, though only three are operational. Interfax described them as among the force's most classified aircraft.
Should a nuclear war break out, the Russian president would board the aircraft, where he or she could potentially order the launch of intercontinental ballistic missiles (ICBMs) and make other strategic decisions.
With no external windows except in the cockpit, the aircraft are designed to have some protection from the effects of a nuclear blast. The US operates a similar fleet of airborne command posts, called E-4B Nightwatch.
In May last year, Russia's Deputy Defence Minister Alexei Krivoruchko said work was under way to upgrade and re-equip the country's Il-80s.
A new generation of airborne command posts are reported to be in development as well.
Russia's Tass news agency reported in October that the Il-80s are scheduled to be replaced by a new model, the Il-96-400M, but did not give a timeframe.
