CHEORWON, South Korea >> South Korea exploded one of its own front-line guard posts today, sending plumes of thick, black smoke into the sky above the border with North Korea, in the most dramatic scene to date in the rivals’ efforts to reduce animosities that sparked last year’s fears of war.

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. The journalists were asked to stay hundreds of meters (yards) away as black smoke enveloped the hilly border area. They later watched soldiers and other workers bulldoze another guard post.

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

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

The guard posts are inside the 248-kilometer (155-mile) -long, 4-kilometer (2.5-mile) -wide border called the Demilitarized Zone. 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 front-line 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. They’ve also halted live-fire exercises along the border.

The deals are among a set of steps they have taken since North Korean leader Kim Jong Un reached out to Seoul and Washington early this year with a vague commitment to nuclear disarmament. The fast-improving inter-Korean ties have raised worries among many in South Korea and the United States as global diplomacy on the North’s nuclear weapons program has produced little recent progress.

Speaking in Washington on Thursday, South Korean Unification Minister Cho Myoung-gyon acknowledged that criticism and offered reassurance that the South would not pursue economic cooperation with the North until there’s “significant progress” on denuclearization.

But he argued that improving inter-Korean relation would help, not hinder that process. He said another summit between South and North Korea could still take place this year. If it happens, Cho said it would break a “glass ceiling” in inter-Korean relations and could pave the way for another U.S.-North Korea summit.

“Chairman Kim’s visit to Seoul and South Korea is something that we have agreed upon, something that we can implement and something that is possible,” Cho said at the Wilson Center think tank.

The two Korean leaders have already met three times this year, most recently in North Korea’s capital in September. But a lack of progress in U.S.-led efforts to get the North to give up its nukes has added to doubts about whether plans for South Korean President Moon Jae-in to host Kim in the South Korean capital during 2018 can be realized.

If Kim, a third-generation hereditary ruler, visits Seoul, he would be the first North Korean leader to do so since the Korean War ended in 1953 with an armistice.