UKRAINE CALLS ON US TO BACK TRIBUNAL TO TRY KREMLIN FOR WAR CRIMESPutin made headlines after he appeared to suggest that he may make changes to Russia’s nuclear deterrence doctrine by employing a first-strike policy, which he referred to as a "disarming strike" — a strategy he said is motivated by the current U.S. deterrence policy.Washington does have a policy that allows it to use a nuclear weapon, not only in retaliation to a nuclear attack but also in response to non-nuclear threats — and so does Moscow, explained Rebekah Koffler, a Russia expert and former Defense Intelligence Agency intelligence officer for Russian Doctrine & Strategy."Based on the existing Russian doctrine, Russia can respond with nuclear strikes, because Engels airbase is the home to nuclear-capable bombers that are considered strategic assets, worthy of defending with nukes," Koffler explained.PUTIN HOLDS COUNCIL MEETING ON DOMESTIC SECURITY FOLLOWING 3 AIRBASE 'EXPLOSIONS'Kyiv has not accepted responsibility for the strikes, but the attacks outside of Ukraine’s borders, where a brutal war has raged for more than nine months, put top defense officials on the alert this week.CLICK HERE TO GET THE FOX NEWS APP"There’s no question that Russia and the U.S. are climbing an escalatory ladder in Ukraine, which may end up in Putin’s use of nuclear weapons," she told Fox News Digital."