North Korea’s military on Tuesday threatened the South with imminent "sledge-hammer" retaliation unless Seoul apologized for anti-Pyongyang protestors burning effigies of its revered leaders.

North Korea's military on Tuesday threatened the South with imminent "sledge-hammer" retaliation unless Seoul apologized for anti-Pyongyang protestors burning effigies of its revered leaders.

South Korea called the North's ultimatum "regrettable" and "absurd" with the defense ministry vowing a tough response to any military provocation.

As North Korea marked the birth of late founding leader Kim Il-Sung on Monday -- a major national holiday -- around 40 protesters in Seoul burned portraits of Kim, his son Kim Jong-Il and grandson and current leader Kim Jong-Un.

Condemning what it described as a "thrice-cursed... monstrous criminal act", the Korean People's Army (KPA) Supreme Command issued an "ultimatum" threatening "just" and immediate action if an apology was not forthcoming.

"Our retaliatory action will start without any notice," it said in a statement carried by the Korean Central News Agency.

"The military demonstration... will be powerful sledge-hammer blows at all hostile forces hurting the dignity of the supreme leadership," it added.

South Korea's defense ministry called Tuesday's ultimatum "regrettable" and promised to retaliate "thoroughly and resolutely" to any provocation.

A Foreign Ministry spokesman said the North's threat was "absurd" and urged Pyongyang to follow a path to dialogue proposed by Seoul and Washington.

"We hope the North will make a wise choice," the spokesman said.

"If the puppet authorities truly want dialogue and negotiations, they should apologize," the KPA statement said.