South Korea's military warned Monday it would immediately retaliate against North Korea's "core command forces" for any attack, after a series of threats from the communist state.

Seoul's armed forces held an unscheduled readiness drill Monday due to a "series of peculiar activities", a defence ministry spokesman said.

Cross-border tension has been high since the North's military on June 4 threatened rocket attacks on the offices of South Korean media outlets for their critical coverage of Pyongyang.

Some of the North's fighter jets flew threateningly close to the border recently, the spokesman said.

Defence Minister Kim Kwan-Jin ordered the drill so the military could "immediately retaliate against sources of provocations and supporting forces as well as core command forces", the spokesman said.

"This is our warning aimed at discouraging the North from daring to provoke us," he said. Yonhap news agency said the drill involved the South's ballistic missile command, front-line artillery units and the air force.

The North's military, in an unusual move, last week listed the coordinates of some media offices and said missile units and other forces had already entered the target information.

It accused them of insulting its regime through coverage of a youth event which saw some 40,000 children tearfully vow loyalty to new leader Kim Jong-Un.

One Seoul outlet likened the activities to the Hitler Youth.

Another media outlet criticised by the North -- JoongAng Ilbo newspaper -- became the victim of a major cyber attack on Saturday, which temporarily paralysed its database servers and website.

Police are investigating Pyongyang's possible involvement in the hacking targeting the JoongAng Ilbo and sister paper the Korea JoongAng Daily, the Daily reported.

"We have never seen a strong attack like this before," the paper quoted Jong Seok-Hwa, chief investigator of the national police force's Cyber Terror Response Center, as saying.

Seoul accused Pyongyang of staging cyber-attacks on websites of major South Korean government agencies and financial institutions in March last year and in July 2009. The North denied the charges.

Cross-border tension has been especially high since Jong-Un took over when his father Kim Jong-Il died last December.

Pyongyang since then has heaped insults on South Korean President Lee Myung-Bak and other conservative leaders, branding them "human scum" attempting to raise tension for political gains.

It has threatened "sacred war" against Seoul for perceived insults.