The move, primarily aimed at Russia, could increase the risk of miscalculation between the two countries.
The US military wants to revamp its nuclear arsenal and develop new low-yield atomic weapons, largely in response to Russian actions in recent years, the Pentagon said in a policy statement released on Friday.
The so-called Nuclear Posture Review outlines the Pentagon's nuclear ambitions under President Donald Trump and is the first time since 2010 that the military has spelled out how it foresees nuclear threats in the coming decades.
The document marks a sobering break from the vision for America's atomic future under Barack Obama, who during a famous speech in Prague in 2009 called for the elimination of nuclear weapons.
Trump last year advocated upgrading US's nuclear arsenal.
My first order as President was to renovate and modernize our nuclear arsenal. It is now far stronger and more powerful than ever before....— Donald J. Trump (@realDonaldTrump) August 9, 2017
While it underscores the administration's concerns about North Korea, Iran and China, the focus falls largely on Russia.
"This is a response to Russian expansion of their capability and the nature of their strategy and doctrine," Defense Secretary Jim Mattis wrote in the introduction to the 75-page document.
"These developments, coupled with Russia's seizure of Crimea and nuclear threats against our allies, mark Moscow's decided return to Great Power competition," he also wrote.
The Pentagon worries Russia assumes America's regular, large-yield weapons are essentially too big to ever be detonated, as their use would likely result in large-scale retaliation and wipe much of humanity off the map.
"There are strong indications that our current strategy posture and capabilities are perceived by the Russians as potentially inadequate to deter them," Greg Weaver, the deputy director of strategic capabilities for the military's Joint Staff, told reporters.
The move some, critics say, could increase the risk of miscalculation between the two countries.
The policy statement drew criticism and raised alarm among advocates of nuclear nonproliferation.
Today Trump's new nuclear doctrine comes out. It lowers the threshold for using nuclear weapons and calls for smaller "more usable" nuclear weapons. It is a dangerous policy and leads us closer to nuclear war. States must join #nuclearban and resist this! #NuclearPostureReview pic.twitter.com/xBZ6bJQ905— Beatrice Fihn (@BeaFihn) February 2, 2018
More reliance on subs and ships
The document says that by having additional smaller nukes, the Pentagon can counter adversaries' "misperceptions" that the US would not respond to another country using its own low-yield bomb.
The new strategy calls for a continuation of the nuclear modernisation programme ordered by Obama that encompasses all pillars of the "triad" -- ground-based intercontinental ballistic missiles, submarine-launched weapons and bombs delivered by plane.
But unlike the Obama strategy, which stressed reducing the role of nuclear weapons, the new policy has a more assertive tone.
Low-yield nuclear weapons, also known as "tactical" nukes, are still extremely powerful and can pack as much destructive punch as the bombs dropped on Hiroshima and Nagasaki at the end of World War II.
America already has a massive nuclear arsenal at its disposal, including 150 B-61 nukes stored across multiple European countries that can be configured for low-yield options.
The new weapons envisioned by the Pentagon would be launchable from submarines or ships, so would not need to be stockpiled in Europe.
They could also get around Russian air defenses more easily.
The bombs would not add to America's nuclear horde, and would instead repurpose existing warheads, but critics say the Pentagon would be going against the spirit of non-proliferation agreements.
"We are on the cusp of a new era of nuclear proliferation," warned Barry Blechman, co-founder of the Stimson Center, a nonpartisan anti-nuclear proliferation think tank in Washington.
"This is the great nuclear danger raised by the new" nuclear policy.
Jon Wolfsthal, a former top advisor to President Barack Obama on arms control, said there was a possibility that it could lead to a miscalculation.
"If we put nuclear weapons on cruise missiles and we launch conventional cruise missiles, how does Russia know that they are conventional?" he said.