With the war of words between the US and North Korea escalating, focus has turned to Pyongyang's missile programme. But just how advanced is it?

North Korea's missile arsenal has progressed over the decades from crude artillery rockets derived from World War II designs to testing what it says are long-range missiles that may be able to strike targets in the US.

Intercontinental missiles

North Korea's latest efforts are focused on building reliable long-range missiles, which would have the potential of reaching the mainland United States.

On 4 July 2017, Pyongyang said it had carried out its first successful test of an inter-continental ballistic missile (ICBM). It said the Hwasong-14 could hit "any part of the world", but initial US estimates put the range as shorter than that.

The US military described it as an intermediate-range missile, but a number of US experts said they believed the missile could reach the US state of Alaska.

On 28 July 2017, North Korea carried out its second and latest ICBM test, with the missile reaching an altitude of about 3,000km and landing in the sea off Japan.

Pyongyang has also displayed two types of ICBMs, known as the KN-08 and KN-14, at military parades since 2012.

Carried and launched from the back of a modified truck, the three-stage KN-08 is believed to have a range of about 11,500km.

The KN-14 appears to be a two-stage missile, with a possible range of around 10,000km. Neither has yet been tested, and the relationship between them and the Hwasong-14 is not yet clear.

Nuclear advances?

Media reports in the US have claimed that Pyongyang has now made a nuclear warhead small enough to fit inside its missiles. While not confirmed, this has been seen as one of the last obstacles to North Korea being a fully nuclear-armed state.

A report in the Washington Post, citing US intelligence officials, suggested North Korea was developing nuclear weapons capable of hitting the US at a much faster rate than expected.

A Japanese government defence paper also said the weapons programme had "advanced considerably" and that North Korea possibly now had nuclear weapons.

Why build ICBMs?

Inter-continental ballistic missiles are seen as the last word in power projection because they allow a country to wield massive firepower against an opponent on the other side of the planet.

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The only real reason to spend the money, time and effort building them is to have the capability to fire nuclear weapons.

During the Cold War, Russia and the United States sought different ways to protect and deliver their missiles, which were hidden in silos, piggybacked on huge trucks or carried by submarines.

All ICBMs are designed along similar lines. They are multi-stage rockets powered by solid or liquid fuel, and carry their weapon payload out of the atmosphere into space.

The weapon payload - usually a thermonuclear bomb - then re-enters the atmosphere and detonates either above or directly on top of its target.

Some ICBMs have a "multiple independently targetable re-entry vehicle", or Mirv.

Image copyright Reuters Image caption The country is highly militarised

This has multiple warheads and decoys, allowing it to strike multiple targets and confuse missile defence systems.

In the Cold War period, the range and potential threat of ICBMs were seen as key to the concept of "Mutually Assured Destruction" or MAD.

MAD supposedly helped maintain peace because neither side could "win" without suffering incalculable damage.

The North's missile milestones

North Korea's own missile programme began with Scuds, with its first batch reportedly coming via Egypt in 1976.

By 1984 it was building its own versions called Hwasongs.

These missiles have an estimated maximum range of about 1,000km, and carry conventional, chemical and possibly biological warheads.

From the Hwasong came the Nodong design - effectively an upscaled Hwasong/Scud with an extended range of 1,300km.

In an April 2016 analysis, the International Institute for Strategic Studies said the missiles were a "proven system which can hit all of South Korea and much of Japan".

More capable missiles followed with the development of the Musudan range, which was most recently tested in 2016.

Estimates differ dramatically on its how far it can fly, with Israeli intelligence putting it at 2,500km and the US Missile Defense Agency estimating about 3,200km. Other sources suggest a possible 4,000km.

Another development came in August 2016 when North Korea announced it had tested a submarine based "surface-to-surface, medium-to-long-range ballistic missile", called the Pukguksong.