The United States is updating its contingency plans for a potential war with Russia for the first time since the collapse of the Soviet Union, according to US officials.

The US Defense Department is re-evaluating its Cold War era military plans following increasing tensions between Washington and Moscow over the conflicts in Ukraine and Syria, the Foreign Policy magazine reported Friday, citing current and former Pentagon officials.

“Given the security environment, given the actions of Russia, it has become apparent that we need to make sure to update the plans that we have in response to any potential aggression against any NATO allies,” says one senior Pentagon official familiar with the updated plans.

“Russia’s invasion of eastern Ukraine made the US dust off its contingency plans,” says Michèle Flournoy, a former undersecretary of defense for policy and co-founder of the Center for a New American Security. “They were pretty out of date.”

This would signal a major departure from post-Cold War US military policy. The policy shift came after Russia’s reunification with Crimea last year and the conflict in Eastern Ukraine.

The Pentagon is also reviewing several hybrid warfare strategies, and even a nuclear attack. “As you look at published Russian doctrine, I do believe people are thinking about use of tactical nuclear weapons in a way that hadn’t been thought about for many years,” says the senior official who spoke on the condition of anonymity.

The military strategy has two dimensions. One focuses on what the US would do as part of NATO if Russia attacks one of NATO’s member countries. The other plan considers US action outside the NATO alliance.

The conflicts in Syria and Ukraine have significantly increased tensions between Russia and the US.

Washington accuses Moscow of orchestrating an “illegitimate referendum to annex Crimea and fueling unrest in eastern Ukraine.

Moscow, however, has repeatedly denied having a role in the Ukraine conflict, despite accusations by Kiev and its Western backers.