The interior ministry said police in the southern city of Taganrog had been alerted that 1 million rubles ($13,600) worth of equipment was stolen from an Ilyushin Il-80 plane at an airfield.

President Vladimir Putin's spokesman Dmitry Peskov described the breach as an "emergency situation" and vowed that "measures will be taken to prevent this from happening in the future."

The interior ministry did not specify what was stolen but said that investigators had been dispatched to the scene.

The port city of Taganrog, more than 1,100 kilometres (700 miles) south of Moscow, is home to Beriev Aircraft Company, a struggling state-controlled enterprise.

Both the interior ministry and Beriev declined to comment when reached by AFP.

But the Kremlin-friendly REN-TV television channel reported earlier this week that radio equipment was taken from an Il-80 plane that was undergoing maintenance at Taganrog.

Thieves opened the aircraft's cargo hatch and shoe and fingerprints were found inside the aircraft, the channel said, adding that Beriev reported the theft to police last week.

The channel said that 12 people had been questioned as part of the probe that was made public on Wednesday.

Citing a source, it said officials with access to the airfield could be behind the high-profile theft.

- 'Classified data leak' -

The robbed plane is one of four flying command centres built to evacuate the president and other top officials and issue commands in case of a nuclear explosion.

Based on the Ilyushin Il-86 jetliner, the first such Russian plane is believed to have flown in 1985.

The aircraft are designed to sustain electromagnetic pulses and are almost windowless to prevent the crew and passengers from being blinded by the effects of a nuclear blast.

Military experts said the theft was no ordinary incident and underscored the logistics of guarding highly classified hardware.

Military expert Mikhail Khodarenok said that the Soviet-era radio equipment from the Il-80 was most likely targeted for its scrap value -- a common occurence in Russia.

Writing in the Gazeta.ru news portal, Khodarenok said the incident was significant because it constituted a "highly classified data leak."

"Heads will roll," said Vasily Kashin, a military expert at Moscow's Higher School of Economics.

But he cautioned it was important not to exaggerate the national security significance of "theft of old Soviet-era metal scrap".

"We don't know the condition of that plane," Kashin told AFP.

Another miliary expert, Pavel Felgenhauer, said the loss of the Soviet-era equipment was likely to have rendered the plane unusable.

Last year, Deputy Defence Minister Alexei Krivoruchko said work was under way to modernise the Il-80s.

The United States operates similar flying command centres which are called E-4B Nightwatch and based on the Boeing 747s.