North Korea's missile and nuclear programme
A series of successful North Korean missile tests has demonstrated its growing power and expertise, stoking tensions with the US.
The developments have been rapid: it conducted its largest nuclear test to date on 3 September and flew its second ballistic missile of the year over Japan on 15 September.
Here's what you need to know about the North's weapons programme and military forces, and the South Korean and American forces in the region which could be involved should conflict occur.
Several missile tests in 2017 suggest that North Korea is successfully developing missiles with the potential to strike long range targets, including possibly on the US mainland.
In May 2017, North Korea test fired a Hwasong 12 missile, which analysts believe could have a range of up to 4,500km, putting US military bases on the Pacific island of Guam well within in striking distance.
But two tests in July of the Hwasong-14 missile demonstrated even greater potential, possibly giving North Korea its first truly intercontinental ballistic missile (ICBM).
Analysts believe this missile could be a two-stage version of the Hwasong-12. Initial estimates suggested a range of about 8,000km, but later studies suggested that it could travel as far as 10,000km if fired on a maximising trajectory, allowing it to potentially hit New York.
In August 2017 the second missile test of the Hwasong 12 travelled over Japan, the first time a ballistic missile has flown over the territory and was described as an "unprecedented threat".
This was repeated on 15 September with a missile that flew a greater distance and reached a higher altitude than the August test, demonstrating an improvement in range for the missile.
The apparent successes of all these tests, in contrast to the multiple failures of the earlier Musudan missiles, have raised questions as to how North Korea's missile programme has improved so rapidly.
Michael Elleman from the International Institute for Strategic Studies (IISS) suggests that Pyongyang may have acquired high-performance liquid-propellant engines from illicit networks in Russia and Ukraine.
Pyongyang has also previously displayed two other purported ICBMs, known as the KN-08 and KN-14. Neither has yet been tested, and the relationship between them and the Hwasong's 12 and 14 is not yet clear.
In another development, American military intelligence now believes that North Korea has also now successfully miniaturised a nuclear warhead to fit inside a missile.
North Korea's 3 September nuclear test was by far the biggest of six nuclear tests carried out since 2006, analysts concluded.
The test triggered a 6.3 magnitude earthquake, ten times more powerful than that recorded in a previous test in September 2016.
Estimates of the latest device's explosive power, or yield, ranged from 100-370 kilotons. A yield of 100 kilotons would make the test six times more power than bomb dropped on Hiroshima in 1945.
North Korea claims this test was its first thermonuclear weapon - the most potent form of nuclear weapon where an atomic detonation is boosted by a secondary fusion process to produce a far bigger blast.
Pyongyang started its nuclear programme in the 1980s. Its first nuclear test - of a device yielding a suspected 0.7kt - was held in 2006.
North Korea has one of the largest standing armies in the world, but much of its equipment is old and obsolete.
Despite this, its conventional forces could still inflict massive damage on South Korea should war break out.
While the official military budget is unknown, a regime policy known as 'songun' prioritises spending on the armed forces over all other aspects of government.
In addition to a standing army of more than one million, and estimated reserves of some five million, North Korea also has around 200,000 special forces troops.
These special forces could be expected to infiltrate south in any conflict, potentially exploiting a semi-secret network of 20-25 large tunnels which span the Demilitarized Zone (DMZ), emerging behind South Korean and American forward lines.
A further potent threat comes from thousands of North Korea artillery pieces and rocket launchers deployed along the border, many dug into reinforced concrete positions.
From here the North could unleash crude yet devastating firepower on South Korea, including the capital Seoul, which is well within range.
Chemical weapons could also be used. In 2012 the South Korean government assessed that North Korea could have between 2,500 and 5,000 tons of chemical weapons, potentially one of the largest stockpiles on Earth.
While South Korea would rely heavily on American assistance in the event of war, South Korean President Moon Jae-in has recently signalled he plans to increase military spending.
American forces in South Korea and the wider region
The United States has had a military presence on the Korean Peninsula since the Korean War, and today South Korea has the third highest deployment of American troops anywhere in the world.
Some 28,000 US troops are stationed across South Korea, the IISS estimates, including almost 9,000 air force personnel . In addition, the US has some 300 M1 Abrams tanks and armoured vehicles deployed.
The US has also installed its THAAD missile defence system at Seongju in South Korea, which would be used shoot down North Korean short and medium range missiles in the event of war.
In the wider region, Japan hosts more US forces than any other nation on Earth with some 47,050 deployed, according to the IISS, the majority being naval personnel.
Yokosuka in Japan is the headquarters of the US Seventh Fleet, which typically has around 20 ships deployed, including the only permanently forward-deployed aircraft carrier strike group led by the USS Ronald Reagan.
Significant US forces are also present on the island of Guam, which is sometimes described as a "permanent aircraft carrier".
Andersen Air Base on Guam hosts the 36th Wing, which operates America's three strategic bomber types the B-52, B1-B Lancer, and B2-Spirit.
In March, it was reported that the US Air Force had deployed an additional 12 B-52s and 12 B-1B aircraft to reinforce its presence on Guam.
North Korea has threatened to fire missiles at the waters around Guam, in response it says to the US flying bomber aircraft over the Korean Peninsula.