NORTH Korea's military has threatened to blow up the Seoul offices of South Korean media outlets following critical coverage of a mass children's event in Pyongyang.

The military general staff, in an unusually detailed statement on the official news agency, said missile units and other forces had fixed the longitude and latitude co-ordinates for several firms' offices in central Seoul.

The statement on Monday named the Chosun Ilbo and JoongAng Ilbo newspapers, a TV channel operated by Dong-A Ilbo newspaper, and the KBS, CBS, MBC and SBS television stations.

The North's military accused conservative South Korean President Lee Myung-Bak of inciting the coverage and called on him to apologise to avert an attack.

"In case dens of monstrous crimes are blown up one after another, the Lee group will be entirely held responsible for this," it said in what it termed an "open ultimatum".

The military accused the media outlets of "monstrous mud-slinging" over their coverage of an event which brought 20,000 schoolchildren to Pyongyang to mark the 66th anniversary of the Korean Children's Union.

Pyongyang's official Korea Central News Agency reported on Saturday that the children pledged their loyalty to new leader Kim Jong-Un.

"The young delegates could not hold back the endless happiness ... amid unchanging longing to remember the benevolent image of respected comrade Kim Jong-Un deep in their hearts," it said.

"Sir Kim Jong-Un, thank you so much for giving us such a great honour," it quoted a 13-year-old schoolchild as saying.

The North's military accused the South's government and media outlets of portraying the event as a propaganda gimmick staged by Pyongyang's leadership.

It complained that the "Lee group is letting loose a string of vituperations describing all these blessings as 'charades'."

Seoul's unification ministry, which handles cross-border affairs, said Pyongyang's latest threat was "completely out of line".

"This... is a significant challenge and provocation to free democracy," said a ministry spokesman. "We are taking this very seriously and urging the North to stop such threats to our media immediately."

Jong-Un took over last December from his deceased father and longtime leader Kim Jong-Il, the second power transfer within the Kim dynasty which has ruled the isolated communist state since its founding in 1948.

The new regime has intensified hostile commentary against the South and threatened "sacred war" as it tries to bolster the young Kim's leadership.

The North's military in April vowed to turn parts of Seoul into ashes, accusing Lee and several media outlets of defaming its leadership.