Tensions between North Korea and South Korea have risen after Pyongyang accused Seoul of fabricating a landmine incident at the border and sent a letter to the UN Security Council. A North Korean howitzer spotted near the frontier has added to concerns over rising military friction.

A Reuters witness at a South Korean observatory overlooking the heavily armed border confirmed on Monday that a North Korean artillery piece was visible near the frontier. South Korea’s Yonhap news agency identified it as a self-propelled 122-mm howitzer, calling its public display rare.

Reuters also observed three North Korean armoured vehicles with their gun barrels pointing south and around 20 to 30 soldiers carrying out fencing work inside the Demilitarised Zone (DMZ).

"It is highly unusual (for North Korea) to expose its self-propelled artillery so conspicuously,” said Lim Eul-chul, a professor at Kyungnam University in Seoul.

"By deliberately displaying it, North Korea is signalling through action, not words, that it intends to enforce its ​border policy through force rather than dialogue or negotiation."

Lim added that the move also warned South Korea against escalating border tensions and could aim to prevent an accidental clash.

North Korean state media KCNA reported that Pyongyang’s UN representative sent the letter on Sunday, accusing Seoul of using the landmine incident to portray North Korea’s border construction as a military incursion. The letter warned that violations of its sovereignty could trigger self-defence measures, adding that South Korea would be "wholly ​accountable for all the ensuing consequences."

The letter responded to South Korea’s submission to the Security Council last week. Seoul said a September 21 explosion in the DMZ was caused by a mine planted by North Korean troops, violating the Korean War Armistice Agreement. The UN Command, which oversees the DMZ, said a North Korean mine across the line violated the armistice. Pyongyang has denied responsibility.

South Korea’s Unification Ministry said on Monday that the letter repeated North Korea’s existing position and sought to justify its border-marking activities. The ministry declined to comment separately on the howitzer’s placement. Seoul has proposed talks with Pyongyang to ease tensions.

North Korea also accused South Korea of firing at its personnel, calling it a violation of the UN Charter. Seoul maintains that its military issues warning broadcasts and fires warning shots when North Korean soldiers cross the Military Demarcation Line.

Yonhap also reported that photographs appeared to show North Korean troops conducting coastal drills near Kaepung county, around 1.5 km from the observatory.

A similar mine incident in 2015 injured South Korean soldiers and led Seoul to resume anti-North Korea loudspeaker broadcasts. The resulting military standoff ended after Pyongyang expressed regret and both sides reached a high-level agreement.