The move is a significant change in US defense posture that has raised concerns it could elevate the risk of a nuclear war.

Critics worry that small nukes would be more likely to be used because they cause less damage, thereby lowering the threshold for nuclear conflict.

But the Pentagon says it is crucial to deterring rivals like Moscow who might assume that, with only large, massively destructive nuclear weapons in its arsenal, the United States would not respond to another country's first use of a small, "tactical" nuclear bomb.

The deployment of the W76-2 low-yield warhead is "to address the conclusion that potential adversaries, like Russia, believe that employment of low-yield nuclear weapons will give them an advantage over the United States and its allies and partners," Under Secretary of Defense John Rood said in a statement.

The W76-2 has an estimated explosive yield of five kilotons, compared to the 455-kiloton and 90-kiloton yields of nuclear warheads already deployed on US submarines, according to William Arkin and Hans Kristensen, writing on the Federation of American Scientists website.

It is also smaller than the 15 kiloton and 21 kiloton atomic bombs US forces dropped on Hiroshima and Nagasaki, Japan near the end of World War II in 1945.

Arkin and Kristensen said the new small warheads have been deployed on the USS Tennessee submarine, patrolling in the Atlantic Ocean.

- Mutual assured destruction -

The Pentagon indicated that it would deploy a small nuclear weapon in its 2018 Nuclear Posture Review.

Behind the development was the US view that Moscow itself was developing tactical nuclear weapons in the expectation it could use them without provoking the massive retaliation that was the basis of the classic nuclear deterrence calculus of the Cold War: "mutual assured destruction."

The Pentagon view was that the Russians, if they found themselves struggling in a conventional war, might use a small nuclear weapon if they thought the US military, with only large nukes at hand, would not retaliate with them.

US forces have had tactical-sized nuclear weapons for many years, but ones that could only be dropped as bombs from aircraft or delivered by cruise missile -- which Russia could more easily defend against.

A low-yield nuclear warhead on a submarine-launched ballistic missile would more likely penetrate Russian defenses, and, in the Pentagon's view, more likely make Moscow, or another adversary like China, think twice before attacking with its small nukes.

The new weapons "strengthens deterrence and provides the United States a prompt, more survivable low-yield strategic weapon," Rood said.

It also "demonstrates to potential adversaries that there is no advantage to limited nuclear employment because the United States can credibly and decisively respond to any threat scenario."

But critics say that, after decades in which the sheer size of nuclear weapons had been seen as a deterrence to their use, a small nuclear warhead could increase that possibility.

"While some argue that this warhead is a response to Russia's so-called 'escalate to de-escalate' strategy that will strengthen deterrence and raise the nuclear threshold, others contend that it will lower the threshold for US use and increase the risk of nuclear war," the Congressional Research service said in a report last month.

- Global stability -

The deployment came after the US withdrawal last year from a Cold War-era arms control pact, the Intermediate-Range Nuclear Forces Treaty, which had aimed to lower the risks of nuclear conflict by banning ground-launched, nuclear-capable ballistic and cruise missiles with ranges between 500 and 5,500 kilometers.

Washington withdrew after it concluded that Russia had already developed a missile, the SSC-8, that violated the treaty.

The Pentagon move also comes as the US and Russia face a decision on whether to renew the 2010 US-Russia New Strategic Arms Reduction Treaty, or "New START," which saw both sides reduce the total number of nuclear warheads they have.

"An extension would show the world that Washington and Moscow are committed to a verifiable, effective arms control agreement for the benefit of global stability," said Brian Sittlow of the Council of Foreign Relations.