SEOUL -- South Korea marked its annual Armed Forces Day on Thursday with a ceremony overshadowed by a standoff with North Korea over a mine explosion that injured three soldiers last month.

Tensions rose after a mine detonated on Sept. 21 in the Demilitarized Zone that separates North and South Korea. Investigations have concluded that the explosive device was placed there by North Korean personnel.

On Wednesday, the United Nations Command announced that on-site checks, conducted along with South Korean authorities, into the explosion in the DMZ turned up an active North Korean antipersonnel mine on the South Korean side of the buffer zone. The UNC said that North Korea had violated the Armistice Agreement between the two sides.

South Korea's Joint Chiefs of Staff held a public briefing on the same day, during which an official strongly called on North Korea to take responsibility for the mine explosion and apologize. On Tuesday, Defense Minister Kang Shin-chul said that North Korea had buried the mine in the DMZ, and that it had not been displaced from another location. Kang said that his country would take unspecified "corresponding measures." Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, vehemently denied that North Korea was behind the mine explosion. In a state media missive on Wednesday, she accused the South of "concocting baseless results" to be used as a pretext for aggression targeting the North. A North Korean village photographed from the South Korean side of the inter-Korean border on Sept. 16.

Lee Jae Myung, South Korea's left-leaning president, now faces a difficult balance, in that he will wish to appear both serious about the danger posed by his nuclear-armed neighbor while also leaving the door open to a resumption of dialogue with the North, a long-held ambition of his.

He favors a dovish approach to North Korea, contending that only through dialogue and cooperation can the two Koreas solve their confrontation that dates back to the Korean War of the early 1950s. In an interview with the New York Times last month, Lee called for the U.S. to consider easing sanctions on North Korea in exchange for a promise from Pyongyang to halt developments of its weapons programs.

That position has opened up Lee to accusations from the conservative opposition that he is soft on the North and has worsened the risks to South Korea's national security by failing to maintain strict military vigilance.

But his address at the Armed Forces Day ceremony on Thursday contained mentions of South Korea's growing defense production capabilities, including the launch on Wednesday of what he called a "state-of-the-art Aegis destroyer" that he stressed was "built with our own technology." He also reiterated a vow that his country will by the mid-2030s have "successfully built nuclear-powered submarines using homegrown technology." There is a path for Lee to appear both tough and open to dialogue, said Peter Ward, an analyst at the Sejong Institute's Center for Korean Peninsula Strategy.

Lee "can send a message to North Korea that violating the DMZ, even in error, is not acceptable. [South Korea] can take measures to make sure the border is better marked and that there is better real-time surveillance," Ward told Nikkei Asia.

The inter-Korean border is not clearly delineated in many places, and there is also the possibility that the North Korean military did place the mines in the location where they exploded, not as a deliberate provocation but because they believed they were on their own side, Ward said.

Also at the ceremony, Lee called the DMZ mine explosion "an unfortunate accident" and said, "My heart remains heavy when I think of the service members who were recently injured." But he did not blame North Korea for the explosion and said, "I hope that the North joins us now on the path of restoring trust and resuming dialogue, following a long interruption." The standoff with North Korea over the explosion will be difficult to resolve, as there is evidence to suggest that the mine was placed there deliberately by North Korea and Pyongyang nevertheless retains "plausible deniability" over the incident, according to Chris Green, a senior consultant on the Korean Peninsula for the International Crisis Group.

Also, the lack of an active channel of dialogue between Pyongyang and Seoul will complicate matters further and raise the risk that miscommunication along the border could flare up into some form of conflict, said Gabriela Bernal, a nonresident fellow at the European Centre for North Korean Studies and founder of Peninsula Dispatch, an independent security, diplomacy and strategy analysis platform.

"The less communication, the more likely such incidents could escalate quickly, since misunderstandings are inevitable if the two sides aren't even speaking to each other," Bernal told Nikkei Asia.