South Korea has blown up one of its frontline guard posts on the border with North Korea, sending plumes of thick, black smoke into the sky in the most dramatic scene to date in the rivals' efforts to reduce tensions that sparked last year's fears of war.

Key points: Dismantlement of guard posts are part of a September agreement

Dismantlement of guard posts are part of a September agreement North Korea is also demolishing guard posts

North Korea is also demolishing guard posts Kim Jong-un inspects test of newly developed "tactical" weapon

Last week, the two Koreas finished withdrawing troops and firearms from some of the guard posts along their border before dismantling them.

The steps are part of agreements signed in September during a meeting between their leaders in Pyongyang, North Korea's capital.

On Thursday, South Korea's military invited a group of journalists to watch the destruction of a guard post with dynamite in the central border area of Cheorwon.

Most of the South Korean guard posts are being destroyed with construction equipment for environmental and safety reasons, but dynamite was used for the this structure because of its location on a high hill where it was difficult to use excavators, the Defence Ministry said.

North Korea is demolishing its guard posts with explosives, according to South Korean media.

The guard posts are inside the 248-kilometre-long, 4-kilometre-wide border called the Demilitarised Zone.

North Korean leader Kim Jong-un and South Korean President Moon Jae-in have met multiple times this year. ( AP: Korea Summit Press Pool )

Despite its name, it's the world's most heavily fortified border with an estimated 2 million land mines planted in and near the zone.

The area has been the site of violence and bloodshed since the 1945 division of the Korean Peninsula, and civilians need special government approval to enter the zone.

The Koreas each agreed to dismantle or disarm 11 of their guard posts by the end of this month before jointly verifying the destruction next month.

South Korea had about 60 posts inside the DMZ guarded by layers of barbed wire and manned by troops with machine guns.

North Korea was estimated to have 160 such frontline posts.

Under the September agreements, the Koreas are also disarming the shared border village of Panmunjom and clearing mines from another DMZ area where they plan their first-ever joint searches for Korean War dead.

Kim inspects testing of new 'tactical' weapon

North Korean leader Kim Jong-un leads the testing of a newly developed tactical weapon. ( KCNA via Reuters )

But concern continues over North Korea's nuclear and missile program despite a summit between Kim Jong-un and US President Donald Trump in June that ended with a vague commitment to nuclear disarmament.

In the first mention by state media of a new development in North Korea's weapons program in months, a report on Friday said Mr Kim had inspected the testing of a newly developed tactical weapon.

"This result today is a justification of the party's policy focused on defence, science and technology, another display of our rapidly growing defence capabilities to the whole region, and a ground-breaking change in strengthening our military's combat capabilities," Mr Kim said.

KCNA said the test was successful.

Mr Kim said that the weapons system tested was one that his father, Kim Jong-il, was especially interested in during his lifetime, and led the development of personally.

Any testing of new weapons is likely to raise tensions with Washington, which has said there will be no easing in international sanctions until North Korea takes more concrete steps to abandon its nuclear weapons and long-range missiles.

Mr Trump on Tuesday criticised what he called "inaccurate" media reports that North Korea had not declared an estimated 20 missile bases and added in a tweet: "I will be the first to let you know if things go bad!"

AP/Reuters