North Korea’s armed forces on Tuesday threatened to attack South Korea if “anti-North Korean” activities did not stop, media reports said.

Pyongyang was angry over a demonstration in Seoul on Monday where a portrait of the North’s former leader Kim Jong Il was burned, South Korea’s Yonhap News Agency reported.

“The supreme command of the Korean People’s Army Tuesday issued an ultimatum to the South Korean puppet group,” reported the Korean Central News Agency, the North’s official news agency.

The military called the burning an “atrocious, unforgivable act.” Pyongyang demanded an apology for anti-North Korean acts and said it would retaliate without warning if they did not stop.

South Korea’s Defence Ministry said it was monitoring the situation.

“It is regrettable that North Korea threatens (the South),” ministry spokesman Kim Min Seok said. “We are closely watching the North Korean military’s move and stand firm against any provocations.” North Korea on Monday marked the 101st anniversary of the birth of state founder Kim Il Sung, grandfather of current leader Kim Jong Un and father of Kim Jong Il.

There were fears that Pyongyang would mark the day with a missile launch as a show of military might despite warnings from the United States that it would be a “huge mistake.” But no test launch was conducted and instead musical shows, group dancing, a sports competition and floral offerings were held during the so-called Day of the Sun.

The muted celebrations were held as US Secretary of State John Kerry and Japanese Prime Minister Shinzo Abe agreed to work closely toward reining in North Korea’s provocative behaviour.

Mr. Kerry said Pyongyang needs to take “meaningful steps” towards denuclearisation to be able to resume six-party talks involving the Koreas, Japan, the US, Russia and China.

The talks aimed at ending North Korea’s nuclear programmes stalled in 2009.

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