Does North Korea have 'H' or 'A' bomb?

North Korea said Wednesday it had successfully tested a miniature hydrogen bomb, which if confirmed, would place it among a small group of countries with such dangerous weapons, AFP reports.

The world's nuclear arsenals have typically comprised two types of warheads: atomic bombs (A-bombs) such as those dropped on Hiroshima and Nagasaki, and much more powerful hydrogen, or thermonuclear bombs (H-bombs).

A third category of "enhanced radiation" (ER) warheads, once dubbed "neutron bombs" was developed using the thermonuclear principle, but they are not considered to be widely deployed at present.

The test, which came just two days before leader Kim's birthday, was initially detected by international seismology monitors as a 5.1-magnitude tremor next to the North's main Punggye-ri nuclear test site in the northeast.

Last month Kim suggested Pyongyang had already developed a hydrogen bomb.

The claim was questioned by international experts and there was continued scepticism over Wednesday's test announcement.

Whether an H-bomb or not, it was North Korea's fourth nuclear test and marked a striking act of defiance in the face of warnings that Pyongyang would pay a steep price if it continued pursuing its atomic weapons programme.

The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions. Their failure to prevent a fourth detonation will see calls for more drastic Security Council action this time around.

International reaction

China said it "firmly opposes" its neighbour's actions while others blasted it as an intolerable provocation that must be punished.

Several governments promised a firm response as tensions soared again in Northeast Asia, with many calling for further action by the United Nations against the North, which is already subject to an array of international sanctions.

The UN Security Council was to hold an emergency session later Wednesday.

South Korean President Park Geun-Hye described the test as a "grave provocation" at an emergency meeting of the country's National Security Council.

"The test is not only a grave provocation to our national security but also a threat to our future... and a strong challenge to international peace and stability," she said, calling for strong sanctions on Pyongyang.

Japanese Prime Minister Shinzo Abe slammed it as "a serious threat to the safety of our nation".

"This clearly violates UN Security Council resolutions and is a grave challenge against international efforts for non-proliferation," he said.

In Washington the White House would not confirm the test, but vowed to "respond appropriately to any and all North Korean provocations".

The foreign ministry of Russia, also a permanent Security Council member, denounced the test as a "flagrant violation of international law and existing UN Security Council resolutions".

"Such actions are fraught with the possibility of aggravating the situation on the Korean peninsula, which already has a very high potential for military and political confrontation," it said.

Other veto-wielding Security Council members Britain and France also joined in the chorus of condemnation.

Speaking in Beijing, Britain's Foreign Secretary Philip Hammond said the test was "a grave breach of UN Security Council resolutions and a provocation".

Paris labelled the move an "unacceptable violation" of UN resolutions and called for a strong reaction from the international community.

Australian Foreign Minister Julie Bishop said her country "condemns in the strongest possible terms" the test, which "confirms North Korea's status as a rogue state and a continuing threat to international peace and security", adding that Canberra would express its concerns to Pyongyang directly and call for stronger UN sanctions. 

Who has nuclear weapons?

Britain, France, China, Russia and the United States, the five permanent members of the UN Security Council, officially have nuclear weapons.

India and Pakistan also have nuclear weapons along with Israel, which maintains a policy of nuclear ambiguity, and North Korea is known to have carried out tests. If North Korea masters the technology needed to produce miniature warheads, it could conceivably use them to arm ballistic missiles able to reach neighbours in Asia and possibly the United States.

The A-bomb

Atomic bombs work on the principle of nuclear fission, where energy is released by splitting atoms of enriched uranium or plutonium encased in the warhead. The first test of an A-bomb took place in July 1945 in New Mexico, United States, and immediately demonstrated the new weapon's awesome power. Hiroshima was destroyed by one A-bomb with a uranium-fuel warhead that had the power of 15 kilotons (0.015 megaton). Nagasaki was destroyed three days later by a plutonium A-bomb of similar power, 17 kilotons, or the equivalent of 17,000 tons of TNT. The Soviet Union tested its first atomic bomb in August 1949 in the desert of Kazakhstan. 

The H-bomb

The hydrogen, or thermonuclear bomb works on the principle of fusion of two nuclei, and generates temperatures similar to those found at the sun's core. When an H-bomb is detonated, chemical, nuclear and thermonuclear explosions succeed each other within milliseconds. The nuclear explosion triggers a huge increase in temperature that in turn provokes the nuclear fusion. The first US test of an H-bomb was on November 1, 1952 in the Marshall Islands, a chain in the Pacific Ocean. A year later the Soviet Union tested its own H-bomb, and the largest blast to date took place on October 30, 1961, when the Soviet "Tsar Bomba" exploded in the Arctic with a force of 57 megatons.

No H-bomb has been used in a conflict so far, but the world's nuclear arsenals are comprised for the most part of such weapons.

"Most of the thermonuclear warheads in service today have so-called 'dial-a-yield' options that allow for low explosive yields (less than 10 kilotons)  with a considerable fraction of that yield derived from fusion reactions, that effectively make them enhanced radiation warheads," notes Shannon Kile of the Stockholm International Peace Research Institute (SIPRI).

ER warheads

They are based on the thermonuclear principle of the H-bomb, but are designed to generate more radiation than energy, thus targeting people while limiting damage to buildings, bridges, and other infrastructures. The warhead was developed to stop tanks and other armoured vehicles by killing or incapacitating their crews.