After a successful missile test, North Korea claims it is now a “full-fledged nuclear power” which is “capable of hitting any part of the world”.

Russia and the US believe this is a slight exaggeration, saying the missile actually had a medium-range and posed no immediate threat to either country.

But the development has scared many about the prospect of nuclear war. So how likely is it?

Who’s got nuclear weapons?

The US and Russia both reduced their nuclear weapon arsenal after the Cold War. But since the 1990s, the speed of this reduction has slowed down.

What’s more, because of constantly improving technology, the potential impact of each warhead is now far greater than it once was.

The Federation of American Scientists (FAS) says: “Comparing today’s inventory with that of the 1950s is like comparing apples and oranges; today’s forces are vastly more capable.

“The pace of reduction has slowed significantly. Instead of planning for nuclear disarmament, the nuclear-armed states appear to plan to retain large arsenals for the indefinite future.”

As far as we know, nine countries have nuclear weapons: Russia, the US, France, China, the UK, Pakistan, India, Israel and North Korea. Between them, they are thought to have around 15,000 nuclear weapons.

Of these, Russia and the US have by far the most. But the FAS says that China, Pakistan, India and North Korea appear to have been increasing their stockpiles, while the others are either reducing the numbers or making no significant changes.

We can’t be sure of exact numbers because of the high level of secrecy, not least in North Korea.

Israel has also refused to confirm or deny its arsenal, but it is widely suspected to have about 80 nuclear warheads and enough plutonium to make many more.

Out of the 15,000 nuclear weapons in the world, the majority are not immediately deployable. A report by the United Nations Institute for Disarmament Research in 2012 estimated that the US, UK, France and Russia had around 1,940 warheads which were “ready for use on short notice”.

The report said the numbers were so high because of “circular (though flawed) logic”.

“US nuclear forces are maintained on alert because Russian nuclear forces are on alert, and vice versa for Russian forces. Put in another way, if nuclear forces were not on alert, there would be no requirement to keep nuclear forces on alert.”

What would happen in a nuclear war?

After the US dropped a nuclear bombs on Hiroshima and Nagasaki in 1945, an initial report said that two-thirds of the people who were within half a mile of the blasts had been killed. People suffered skin burns up to two miles away.

The final death toll of Hiroshima alone is now estimated to be between 66,000 and 150,000.

That was more than 70 years ago, though. Nuclear weapons today can be many, many times more powerful.

And that’s to say nothing of the economic, political and social consequences, which could potentially be monumental.

Bill Perry, a nuclear weapons expert who served as President Clinton’s Secretary of Defense spoke to Vice earlier this year. He said a worst case scenario would now mean nothing short of a total Armageddon.

“An all-out general nuclear war between the United States and Russia would mean no less than the end of civilisation,” Perry said. “That’s not being dramatic; that’s not being hyperbolic. That’s just what would happen.”

How likely is a nuclear war?

The world survived the Cold War without nuclear weapons being used. And with the main two nuclear powers reducing their arsenals, it might be tempting to think the risk is reducing.

But global security threats are very different to what they once were. And one bomb could lead to retaliation strikes.

Bill Perry said he believes the most likely scenario for a nuclear attack would be if a terrorist group got hold of a small amount of enriched uranium, allowing them to make an improvised nuclear bomb.

“Of all of the nuclear catastrophes that could happen, this is the most probable,” he said. “I think I would say it’s probably an even chance that this will happen sometime in the next ten years.”

He added: “We have the possibility of a regional nuclear war, between Pakistan and India, for example. Even if they used only half of their nuclear arsenal, those bombs would put enough smoke in the air – enough dust in the air – that will go up and settle in the stratosphere and then distribute itself around the planet and would block the rays of the sun for years to come.

“It could be millions of people who die from that alone.”