After weeks of speculation that North Korea was planning a third nuclear test, a North Korean foreign ministry spokesman said Saturday that Pyongyang had no such plans "at present" but accused South Korea of trying to force the regime to do so.

"We'll judge them by their actions, rather than their words," US State Department spokesman Victoria Nuland told reporters in Washington.

"It's a good thing, obviously, that they are saying better things, but we continue to call on them to fulfill their international obligations to refrain from any provocative activity, including provocative rhetoric with regard to their neighbors," she said.

Satellite pictures have suggested that North Korea is developing its nuclear capabilities and some analysts believe that young leader Kim Jong-Un needs to prove his mettle with a military test after a rocket launch flopped in April.

North Korea fired the rocket -- in what it called an unsuccessful satellite launch -- just weeks after signing an agreement with the United States in which it pledged to freeze any missile or nuclear tests or uranium enrichment.

The United States considered the launch a veiled missile test and suspended a plan to offer food aid to the impoverished country that would have been geared toward hungry young children and pregnant women.

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.