The world's nuclear arsenals are comprised mainly of two types of warheads - atomic bombs, also called A-bombs, and the more powerful hydrogen or H-bombs.
North Korea, escalating its war of rhetoric with the United States, on Friday hinted it may explode an H-bomb over the Pacific, having already carried out underground tests of atomic and hydrogen bombs.
Here is a rundown on both types of weapon.
The A-bomb
This weapons have only been used twice in conflict, when the United States bombed Japan in the final days of World War II, although they have been tested several hundred times.
Atomic bombs work on the principle of nuclear fission where energy is released by splitting atoms of enriched uranium or plutonium encased in a warhead.
The first-ever explosion of an A-bomb was in a test in the deserts of the US state of New Mexico on July 16, 1945 - the culmination of the secretive Manhattan project to develop such a weapon in the belief that Nazi Germany was doing the same.
On August 6, the United States dropped an atomic bomb on the southern Japanese city of Hiroshima, killing 140,000 people, according to estimates.
Three days later, a second bomb devastated Nagasaki, killing an estimated 74,000 people. Japan surrendered, bringing World War II to an end.
These bombs produced an explosive yield of roughly 20 kilotons, the equivalent to 20,000 tons of TNT.
The shock wave demolished buildings of reinforced concrete and the intense heat vaporised people near the centre of the blast. Others were badly burned or succumbed to radiation-related illnesses weeks, months or years later.
The Soviet Union was the second country to test an atomic bomb in 1949; Britain became the world's third nuclear power with a test in 1952.
China, France, India, North Korea and Pakistan are also confirmed to possess nuclear weapons. Israel is considered to be an undeclared nuclear power, refusing to confirm or deny that it has such weapons.
The H-bomb
Many times more powerful than the atomic bomb, the hydrogen or thermonuclear bomb works on the principle of the fusion of isotopes of hydrogen and generates temperatures on the order of those found at the sun's core.
While no H-bomb has been used in a conflict so far, the world's nuclear arsenals are comprised for the most part of such weapons.
The bomb has a two-stage process with a nuclear explosion triggering a huge increase in temperature that in turn provokes nuclear fusion, setting off a powerful explosion.
The US army tested the first H-bomb in 1952 in an explosion that was almost 700 times more powerful than an atomic bomb.
A year later the Soviet Union tested its own H-bomb. In 1961 it carried out the most powerful blast to date, exploding the "Tsar Bomba" in the Arctic with a force of around 57,000 kilotons (57 megatons).
North Korea said it tested a miniaturised H-bomb in January 2016, although scientists said the six-kiloton yield achieved then was far too low for a thermonuclear device.
It said that its September 3 nuclear test, its sixth, was also of a hydrogen bomb. The underground blast triggered landslides in the detonation area.