Russia’s Su-57 PAK-FA Stealth Fighter: A "Nuclear Strike" Aircraft?
The Su-57 could potentially supplant the Su-34 Fullback bomber—which is Russia’s current nuclear-capable strike aircraft—for intermediate range missions against heavily defended airspace.
Russia’s Sukhoi Su-57 PAK-FA fifth-generation stealth fighter is listed in the Trump Administration’s new Nuclear Posture Review (NPR)  as a developmental dual conventional and nuclear capable strike aircraft. If the NPR is correct, the Su-57 could potentially supplant the Su-34 Fullback bomber—which is Russia’s current nuclear-capable strike aircraft—for intermediate range missions against heavily defended airspace.
Indeed, the NPR contends that Russia is continuing to modernize its arsenal of non-strategic nuclear arsenal of roughly 2000 nuclear warheads.
“Russia is modernizing an active stockpile of up to 2,000 non-strategic nuclear weapons, including those employable by ships, planes, and ground forces,” the NPR reads. “These include air-to-surface missiles, short range ballistic missiles, gravity bombs, and depth charges for medium-range bombers, tactical bombers, and naval aviation, as well as anti-ship, anti-submarine, and anti-aircraft missiles and torpedoes for surface ships and submarines, a nuclear ground launched cruise missile in violation of the 1987 INF Treaty, and Moscow’s antiballistic missile system.”
The NPR posits that the Russians place a premium of nuclear weapons due to Moscow’s “de-escalation” doctrine, which is why the administration is seeking to build new types of American non-strategic nuclear arms.
“These supplements will enhance deterrence by denying potential adversaries any mistaken confidence that limited nuclear employment can provide a useful advantage over the United States and its allies,” the NPR states. “Russia’s belief that limited nuclear first use, potentially including low yield weapons, can provide such an advantage is based, in part, on Moscow’s perception that its greater number and variety of non-strategic nuclear systems provide a coercive advantage in crises and at lower levels of conflict.”
Many Russia experts such as Olya Oliker have cast doubt upon  the notion that Moscow has lowered its nuclear threshold. Oliker notes that Russian military strategy documents from 2010 actually tighten the Kremlin’s policies on the use of nuclear weapons. Indeed, most experts on Russian nuclear weapons such a Nikolai Sokov —a former Soviet and Russian arms control negotiator—believe that Russia is reducing its dependence on non-strategic nuclear arms. 
Nonetheless, it is true that Russia maintains a significant non-strategic nuclear arsenal and a significant portion of those weapons can be delivered by air—even if not specifically by the Su-57 as posited in the NPR. “We do have nuclear bombs for tactical aircraft and air launched tactical nuclear missiles as well. And there are ALCMs  [air-launched cruise missiles] under development that will be used by tactical aircraft,” Vasily Kashin, a senior fellow at the Center for Comprehensive European and International Studies at Moscow's Higher School of Economics told The National Interest. “But I do not remember Su-57 being specifically mentioned.”
Russia’s X-50 air launched cruise missile might fit into the Siu-57’s weapons bays, Kashin said. However, there is no official word from the Russian Defense Ministry. “It is possible, even likely but not confirmed yet,” Kashin said.
Right now, however, the main Russian threat nuclear air delivery platform is the Sukhoi Su-34. That will probably remain the case for some time to come.