In yet another troubling sign for America’s tense relations with Russia, U.S. intelligence officials say Putin has began construction on new nuclear-proof underground bunkers.

Russia began building ‘dozens’ of underground bunkers across the country several years ago, the Washington Free Beacon reported citing US officials.

The U.S. and Russia have been working together to get rid of their nuclear weapons since the end of the Cold War, but the revelation coupled with Russia’s recent surge in missile production suggests they are preparing for conflict.

‘Russia is getting ready for a big war which they assume will go nuclear, with them launching the first attacks,’ Mark Schneider, a former Pentagon nuclear policy official told the Free Beacon. ‘We are not serious about preparing for a big war, much less a nuclear war.’

Few details about the new nuclear bunkers have been released, but Russian state-run media says they are being built in Moscow as part of a new national security strategy.

Russia built several similar underground bunkers during and right after the Cold War both in Moscow and in the Ural Mountains.

Russia’s new nuclear defense strategy is reportedly costing the country billions, and there are questions about whether some of that money may have come from U.S. aid.

Military experts say the U.S. may respond to the new threat by developing deep-penetrating nuclear weapons that would reach the depths of the new bunkers.