The devastating tolls of a new Korean war

Now as well as the South's seat of government, the city is a gleaming hub of technology, K-pop, and plastic surgery, home to 10 million people with even more again in the surrounding area.

But they remain within range of the North's vast artillery forces -- the Demilitarized Zone divides the peninsula only an hour's drive north of the capital -- presenting a tempting target in the event of a new conflict with nuclear-armed Pyongyang.

Even if a second war remained conventional, analysts say tens of thousands of South Koreans would die just on the first day.

Tensions have spiked this year with the North carrying out its sixth atomic test and launching missiles that appear to bring the US mainland within range, and its leader Kim Jong-Un trading threats of war and personal insults with US President Donald Trump.

Trump is due in Seoul next month as part of his Asian tour.

When strains are high "anything can become a miscalculation scenario" and quickly escalate into a larger scale confrontation, said Van Jackson, a former US Department of Defense strategist who now teaches at Victoria University of Wellington.

'Sea of fire'

North Korea is estimated to have some 10,000 artillery pieces and at least 500 short-range missiles stationed along its border with the South, many hidden in caves, tunnels and bunkers.

It has 1.1 million ground troops, Seoul estimates, 70 percent of them stationed within 100 kilometres of the DMZ.

The majority of the arsenal is Soviet- or Chinese-built, rusty and ageing, according to a 2015 report from the US Department of Defense.

But analysts say Pyongyang -- which habitually threatens to turn Seoul into a "sea of fire" -- would try to inflict as much damage as possible in the first hours of any conflict.

Pyongyang has at least 700 170-millimetre guns and 240-millimetre multiple-rocket launchers that can reach Seoul, according to a 2012 study by the Nautilus Institute for Security and Sustainability in California.

Their reliability is in doubt and standard practice would be to fire only about two thirds of them at once, it said, and not all the rounds would explode -- around a quarter of the 170 shells in Pyongyang's 2010 barrage of Yeonpyeong island in the South failed.

Even so, the Nautilus study estimated around 65,000 civilians in Seoul would die on the first day of a conventional North Korean attack, most in the first three hours, rising to 80,000 after one week.

But author Roger Cavazos said: "They can kill many tens of thousands of people, start a larger war and cause a tremendous amount of damage before ultimately losing their regime."

US and South Korean troops would counter-fire within minutes and the fatalities would slow rapidly as civilians took cover in the thousands of underground shelters across the city.

Counter-attacks would destroy about one percent of Pyongyang's artillery every hour -- eliminating almost a quarter after a day -- and most of the fighting would be over in four days.

Apocalyptic weapons

According to Seoul's 2016 defence white paper, Washington would deploy a monumental level of force to the peninsula in the event of war: 690,000 troops, 160 vessels and 2,000 aircraft.

That would be in addition to the 28,500 US troops already stationed in the South, and Seoul's 625,000-strong military -- which would come under US command under the allies' mutual assistance treaties.

They would have overwhelming technological superiority over the North, and all scenarios end with Pyongyang's defeat -- but at what cost?

The North Korean leadership has apocalyptic weapons it could unleash if it finds itself on the brink of annihilation.

"They have a whole tool box they can choose from and they will use them in whatever combination" to their advantage, said Daniel Pinkston, a security analyst at Troy University in Seoul.

Experts believe Pyongyang has around 14 to 18 nuclear warheads, potentially rising to as many as 100 by 2020.

North Korea habitually devotes nearly 25 percent of its gross domestic product to defence, the US Defense Department estimates, with much of the money going to its nuclear and missile programmes.

Estimates vary for the yield of its latest test blast in September -- its most powerful yet, and which it said was a hydrogen bomb -- but both the respected 38 North website and the Norwegian geoscience foundation Norsar put it at 250 kilotons.

According to Nukemap, a computer simulation of atomic attacks, if North Korea detonated a warhead of that size 5000 feet over Seoul, more than 660,000 people would be killed instantly.

And in the event of a similar atomic counter-attack by the US, the figure for Pyongyang is even higher: 820,000.