While the Trump administration says the stance in its nuclear posture review is merely a “clarification” of previous policy, critics say it marks a radical departure.
Alexandra Bell at the Centre for Arms Control and a former non-proliferation official at the state department said: “I think it’s a dangerous and destabilising break with 40 years of bipartisan efforts to reduce nuclear threats around the world.”
She argued that it was dangerous to broaden the circumstances under which the US may use nuclear weapons.
But the Pentagon insists the new measures will allow diplomats to work “from a position of strength”.
US defence Secretary Jim Mattis said in the document: “Our goal is to convince adversaries they have nothing to gain and everything to lose from the use of nuclear weapons.”
He added that more usable so-called tactical nuclear weapons would deter adversaries and raise the threshold for enemy attack.
The posture review plans to re-introduce a nuclear sea-launched cruise missile that was retired under the Obama administration.
It also seeks to equip submarines with lower-yield weapons with power that is up to the equivalent of the nuke dropped on Hiroshima in 1945.
The Trump administration is preoccupied with the accelerating nuclear threat posed by North Korea, but experts say the review is aimed at rising threat from China and Russia.
The Pentagon is focused on maintaining the US’s military advantage as the best means to deter rivals.
Donald Trump has repeatedly argued for large-scale military build-up and called for modernisation of nuclear capability.
Before he took office, he said: “Let it be an arms race.”
He also bragged that he has a “bigger and more powerful” nuclear button than North Korea.
Deputy defence Secretary Patrick Shanahan said the effort would deliver a “modern and credible” deterrent.
He argued new weapons would break no treaties, would not increase the size of the nuclear stockpile and would help deter nuclear attack.
Mr Shanahan was argued the new stance “lowers the risk of nuclear use by anyone”.
But Joseph Cirincione at nuclear policy think-tank Ploughshares Fund said the new approach was “a recipe for locking-in for the next two generations into the cold war architecture that previous administrations have been trying to disassemble.”