The official responsible for keeping Russia's nuclear arsenal safely under lock and key has revealed that lone "terrorists" have twice tried to break into warehouses containing atomic warheads since the collapse of the Soviet Union.

Although Colonel-General Igor Valynkin, head of the defence ministry's shadowy 12th Directorate, did not identify the would-be nuclear thieves, official sources have been quoted in the Russian media as saying they were Chechens. In both cases the men were intercepted by "mobile units" tasked with protecting nuclear storage facilities and arrested by the FSB security service.

Col-Gen Valynkin said Chechen separatists posed the greatest threat to Russia's nuclear arsenal. "We get special information about their plans regarding our nuclear facilities from the FSB and we use this information to urgently take the necessary security measures at the facilities concerned," he said.

He said the two attempted break-ins occurred in 2002 and 2003. Defence ministry sources told the daily Nezavisimaya Gazeta that the incidents were in the Saratov region, where there are two nuclear storage facilities. In both cases Chechens were caught assessing security and trying to get access to warehouses containing warheads.

Viktor Ilyukhin, an MP and deputy chairman of the Russian parliament's security committee, warned that Chechens have inside knowledge. "Before the start of what is called the civil war in the north Caucasus many Chechens served in the armed forces, in the interior ministry troops, and many have experience of guarding our crucial facilities," he told Ekho Moskvy Radio. "Their location is not in any way a secret for the Chechens."

Though Russia is scrapping many of its warheads under bilateral agreements with the United States, its arsenal remains large. There are some 60 warehouses scattered across the country, containing some 1,000 strategic nuclear warheads, and just under 5,000 tactical nuclear weapons, including portable warheads known as "nuclear briefcases". Each month three or four armoured trains containing warheads trundle across the country, and according to the CIA such trains have attracted the attention of Chechen terror groups. In its annual report to Congress, the CIA noted that representatives from two Chechen groups had been seen hanging around at railway junctions outside Moscow trying to get information on the special trains.

Domestically, however, Col-Gen Valynkin's revelation about the attempted break-ins has been interpreted as an effort to secure greater American funding for Russia's cash-strapped nuclear storage facilities.

It is not the first time that Chechens have been accused of trying to get their hands on nuclear weapons. Earlier this year, the UK-based oligarch Boris Berezovsky said Chechen separatists had asked him if he wanted to buy "a nuclear briefcase". The FSB dismissed his claim as nonsense.

Russia insists it has elaborate measures in place to prevent nuclear theft. Guards are carefully vetted and must take regular lie detector tests, and storage facilities are allegedly ringed by hidden motion sensors for miles around. Col-Gen Valynkin said: "Today I can say that it is difficult for terrorists to penetrate our nuclear facilities. Nobody can give a 100 per cent guarantee ... but I guarantee 100 per cent that we securely protect nuclear weapons."