United States remains concerned about nuclear weapons.

• Author: Insinna, Valerie

* The number of nuclear weapons in circulation worldwide has been slowly but steadily declining in recent years because the United States and Russia are scaling back their nuclear arsenals.

Though fewer state-owned strategic warheads exist and the risk of global nuclear war is much more remote than it was during the Cold War, the U.S. nuclear employment strategy acknowledged an increased risk of nuclear attack.

"Today's most immediate and extreme danger remains nuclear terrorism," the strategy, published June 19, stated. "Al-Qaida and their extremist allies are seeking nuclear weapons. We must assume they would use such weapons if they managed to obtain them."

"Today's other pressing threat is nuclear proliferation, in particular Iran and North Korea. The United States wants to keep nuclear weapons from Iranian hands and disavows the legitimacy of North Korea's nuclear arms," the strategy said.

The U.S. nuclear employment strategy also committed to remaining a nuclear power "as long as nuclear weapons exist" in the world.

As a result of a decades-long nuclear arms race, Russia and the United States have by far the largest nuclear stockpiles. Since both nations have agreed to incrementally reduce the number of deployable warheads in their arsenals, the number of nuclear weapons worldwide is steadily declining. The arsenals of other legally recognized nudear states are considerably smaller.

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"Of the five legally recognized nuclear weapon states, only China appears to be expanding the size of its nuclear arsenal," according to the 2014 World Nuclear Forces Report by the Stockholm International Peace Research Institute (SIPRI). The five states recognized by the Nuclear Non-Proliferation Treaty (NPT) are China, France, Russia, the United Kingdom and the United States. The remaining nuclear powers are Israel, India, Pakistan and North Korea, which was the last to join the "nuclear club" in 1998.

Together, the first eight nations possess 4,400 operational nuclear weapons as of Jan. 1, said the SIPRI report. The institute omitted North Korea from the totals because no one is certain how many warheads it has or how many are deployable, though the institute estimated the number between six and eight.

SIPRI estimated that if all nuclear warheads are counted--working, stockpiled, in storage and those slated to be dismounted--then there are about 17,270 nuclear weapons worldwide, excluding North Korea.

Russia remains the only peer...