Explained: What is Demilitarised Zone - the world's last Cold War frontier?

Donald Trump and Kim Jong Un could shake hands in the Demilitarized Zone that divides the Korean peninsula on Sunday if the North's leader takes up the US president's surprise offer to meet.

Trump was to visit the DMZ -- widely referred to as the world's last remaining Cold War frontier -- with South Korean President Moon Jae-in, a day after extending his invitation to Kim on Twitter.

Here are some questions and answers about the Demilitarized Zone, which is based on the positions held at the end of the 1950-53 Korean War, where South Korean forces backed by a US-led UN coalition fought to a standstill with North Korean and Chinese troops.

The four-kilometre-wide DMZ runs for 250 kilometres across the Korean peninsula, around 50 kilometres north of Seoul and 200 kilometres south of Pyongyang.

At its centre is the Military Demarcation Line (MDL), where the front line lay when the ceasefire stopping Korean War hostilities was signed in 1953.

Under the agreement both sides agreed to pull back their forces 2,000 metres.

To the south, Seoul has established a further buffer zone of varying width where civilian access is restricted.

A barrier separating North and South, heavy weaponry is banned from within the DMZ. Patrols are allowed but cannot cross the MDL and no more than 1,000 people from each side are permitted inside the zone at any one time. It is also littered with minefields.

The areas immediately outside it are some of the most highly fortified places on earth, bristling with artillery, military camps, and more minefields.

With the DMZ a "no man's land" and minimal human presence for more than half a century, much of the zone itself is lush forest, renowned as an ecological refuge for rare species of flora and fauna whose habitat elsewhere has been destroyed by development.

An Asiatic black bear was photographed there last October, according to Seoul's environmental ministry.

Within the zone, watchtowers poke up from hilltops, and barbed-wire fences line its edges.

The US and South Korea have been in a security alliance for decades and a trip to the DMZ is something of a ritual for visiting US leaders.

Then president George W. Bush went in February 2002, a month after he named North Korea as part of his "axis of evil".

The last to go was Barack Obama in 2012, and US Vice President Mike Pence went to the border in April 2017 amid heightened tensions with the reclusive state.

President Donald Trump tried to visit seven months later but his helicopter was forced to turn back due to heavy fog.

Most notably, South Korean President Moon Jae-in and North Korean leader Kim Jong Un held two of their three summits last year at Panmunjom, a "truce village" within the DMZ.