The Marshall Islands filed the lawsuits on Thursday in the United States and The Hague, accusing the countries of failing in their obligation to negotiate nuclear disarmament.
The U.S. State Department defended the U.S. record and said its stockpile of nuclear weapons had been cut by 80 percent since the Nuclear Non-Proliferation Treaty took effect in 1970.
"The U.S. is dedicated to achieving the peace and security of a world without nuclear weapons, consistent with our obligations under the Treaty on the Non-Proliferation of Nuclear Weapons," the State Department said in a statement.
"We have a proven track record of pursuing a consistent, step-by-step approach to nuclear disarmament - the most recent example being the New START Treaty," the State Department said, referring to a 2010 nuclear arms reduction agreement with Russia.
The Nuclear Age Peace Foundation, a U.S.-based non-partisan advocacy group supporting the action by the Marshall Islands says the United States plans to spend an estimated $1 trillion on nuclear weapons in the next three decades and currently possesses nearly half of the world's 17,300 warheads.
The Marshall Islands, a group of 31 atolls, was occupied by Allied forces in 1944 and put under U.S. administration in 1947.
The Nuclear Age Peace Foundation said the United States conducted 67 nuclear weapons tests in the Marshall Islands from 1946 to 1958 and islanders were still plagued by the health and environmental effects.
The tests included "Castle Bravo" detonation of a 15-megaton hydrogen bomb on Bikini Atoll in 1954 - the largest the U.S. has ever conducted.
The detonation was estimated to be 1,000 times more powerful than the bomb that destroyed the Japanese city of Hiroshima in 1945, producing an intense fireball followed by a 20-mile-high (32 km) mushroom cloud and widespread radioactive fallout.
In the suits, the Marshall Islands accused all nine nuclear-armed states of "flagrant violation of international law" for failing to pursue the negotiations required by the 1968 Nuclear Non-Proliferation Treaty.
The lawsuits state that Article VI of the NPT requires states to negotiate "in good faith" on nuclear disarmament.
The Nuclear Age Peace Foundation said the five original nuclear weapons states - The United States, Russia, Britain, France and China - were all parties to the NPT, while the others - Israel, India, Pakistan and North Korea - were "bound by these nuclear disarmament provisions under customary international law."
The suit against the United States says that it is not aimed at seeking compensation for the testing in the Marshall Islands, which became an independent republic in 1986.
But it says Washington should take "all steps necessary to comply with its obligations ... within one year of the date of this Judgment, including by calling for and convening negotiations for nuclear disarmament in all its aspects."
Under agreements between the United States and the Marshall Islands, a Nuclear Claims Tribunal was established to assess and award damages to victims of the nuclear tests. But it has never had the cash to compensate fully for the damage done.