The U.S. Defense Department said on Tuesday the Navy had fielded a low-yield, submarine-launched ballistic missile warhead, something the Pentagon believes is needed to deter adversaries like Russia but which critics say lowers the threshold for using nuclear weapons.

Low-yield nuclear weapons, while still devastating, have a strength of less than 20 kilotons. The atomic bomb dropped on Hiroshima, in August 1945, had about the same explosive power.

"This supplemental capability strengthens deterrence and provides the United States a prompt, more survivable low-yield strategic weapon," John Rood, the undersecretary of defense for policy, said in a statement.

"(It) supports our commitment to extended deterrence; and 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," Rood added.