The US-led United Nations Command which oversees the armistice that ended the 1950-53 Korean War, said analysis of the mine debris showed them to be North Korean “wooden box” landmines placed on a known South Korean patrol path.

“The further Ulchi Freedom Guardian joint military exercises are intensified, the strongest military counteraction the DPRK will take to cope with them”, a spokesman for North Korea’s national defence commission said on Saturday.

The North is now an “invincible power equipped with both latest offensive and defensive means… including nuclear deterrence”, the commission said, adding that only by dropping its “hostile” policies against North Korea could the US “ensure the security of its mainland”.

This came hours after Pyongyang threatened “indiscriminate strikes” against the South for resuming propaganda broadcasts along the border, in response to a land-mine blast inside the demilitarised zone that was blamed on the North. Pyongyang denied involvement.

North Korea has switched to a new time zone, turning its clocks back by 30 minutes to mark its liberation from the Japanese at the end of the second World War.

Failure to halt the broadcasts would result in military action “to blow up all means for “anti-north psychological warfare”, along the front, it warned.

“We’re sure they have been North Korean landmines planted with an intention to kill by our enemies who sneaked throughout the army border”, stated Kim Min-Seok, a army spokesman within the South. Last year, North Korean troops opened fire after South Korean activists floated balloons carrying propaganda leaflets across the border.

Additional reporting from The Associated Press.

North Korea also has a different way of counting the years than the West. Instead of 2015, this is Juche 104 on North Korean calendars, which begin from the birth of the country’s founder and first president, Kim Illinois Sung.

The incident comes at a delicate time, with each Koreas getting ready to have fun the 70th anniversary on August 15th of the peninsula’s liberation from Japanese rule.

“Japan should have made an explicit statement on the nature of the war of militarism and aggression and its responsibility on the wars, made a honest apology to the people of victim countries, and made a clean break with the past of militarist aggression, rather than being evasive on this major issue of principle”, she said.

It could also create confusion in messages exchanged between their militaries.