South Korea and the United States will soon sign a new plan on countering any North Korean attacks, Seoul said Wednesday, amid international wariness over the abrupt leadership transition in Pyongyang.

"We believe there remains a possibility of provocations by the North during the power succession to Kim Jong-Un," deputy defence minister Lim Kwan-Bin told reporters.

The ministry said the South Korean military, in response to any attack, would ensure "the enemy threat, the source of the provocation and its supporting forces are completely removed".

The North has hailed Jong-Un as "great successor" and appointed him military chief since his father and longtime leader Kim Jong-Il died suddenly on December 17.

Hopes that cross-border tensions might ease have not so far materialised and some analysts believe the untested son, aged in his late 20s, may try to bolster his credentials by staging a limited border incident.

The new regime has already vowed retaliation against Seoul for alleged disrespect during the mourning period for Kim, and vowed never to deal with its current conservative government.

More than 100,000 people rallied Tuesday in Pyongyang in support of Jong-Un, the North's state media reported. It also released footage of his visit Sunday to an armoured division.

The South's defence ministry, in a policy document for 2012, said the allies would sign the joint counter-provocation plan this month, as agreed last October.

US and South Korean troops already hold regular annual joint exercises.

"Once the joint operational (counter-provocation) plan is signed, we will engage in more exercises that will help us execute it," Lim said. "It will specify how such exercises should be held."

The two Koreas have remained technically at war since their 1950-53 conflict ended only with a ceasefire. The United States has based troops in the South ever since and now has 28,500 in the country.

Cross-border tensions have been high since the South accused the North of torpedoing a warship with the loss of 46 lives in March 2010.

Pyongyang denied involvement but eight months later shelled an island near the tense Yellow Sea border and killed four South Koreans.

South Korea has since strengthened troops and weaponry on its "frontline" islands.

In Washington, the US State Department said Tuesday that the North's stated refusal to engage with South Korea bodes ill for efforts to revive six-party talks on Pyongyang's nuclear disarmament.

"That's not going to be conducive to getting back to the table," said spokeswoman Victoria Nuland.

The North said last week it would never have dealings "with the Lee Myung-Bak group of traitors", in a reference to the South's president.

Nuland said the North should improve ties with the South and show its commitment to denuclearisation before the six-party talks can resume.

The talks -- chaired by China and involving the two Koreas, the United States, Japan and Russia -- have been at a standstill since the last round in December 2008.