North Korea's military Monday threatened rocket attacks on the Seoul offices of South Korean media outlets for their critical coverage of a mass children's event in Pyongyang.

The military general staff, in an unusual move, listed the coordinates of some of the offices and said missile units and other forces had already targeted the buildings.

It accused conservative South Korean President Lee Myung-Bak of inciting hostile media 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," the military said in what it termed an "open ultimatum" carried by Pyongyang's official news agency.

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

The children pledged loyalty to new leader Kim Jong-Un, the North's news agency reported Saturday.

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

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

It took particular exception to comments on the Dong-A TV channel, which likened the North's activities for young people to the Hitler Youth.

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.

That threat prompted police to deploy officers to guard some media outlets.