North Korea Wednesday announced plans for a top leadership meeting and launched a tirade against South Korea's new president, as heightened military alert levels frayed nerves on the tense border.

State media said the politburo of the ruling Workers' Party would meet sometime in the next few days to discuss an unspecified "important issue" and make a "drastic turn".

The military remains the ultimate powerbroker in North Korea, although analysts say leadership changes undertaken by the country's new young leader Kim Jong-Un have given the party added muscle.

The politburo is meant to convene once a year.

News of the meeting came a day after North Korea's military put its "strategic" rocket units on a war footing, with a fresh threat to strike targets on the US mainland, Hawaii and Guam, as well as South Korea.

Military tensions on the Korean peninsula have been at an elevated level for months, following the North's long-range rocket launch in December and the third nuclear test it carried out last month.

Both events triggered UN sanctions that infuriated Pyongyang, which has spent the past month issuing increasingly threatening statements about unleashing an "all-out war" backed by nuclear weapons.

On Wednesday the North's Committee for the Peaceful Reunification of Korea, a state body in charge of propaganda and inter-Korean affairs, launched a scathing verbal attack on South Korea's new president, Park Geun-Hye.

It accused Park, who was only sworn in a month ago, of slander and provocation after she made a speech warning the North that failure to abandon its nuclear weapons programme would result in its collapse.

"If she keeps to the road of confrontation... she will meet a miserable ruin," the committee said.

In Seoul some analysts suggested the North was fast running out of threats and targets for its invective as it sought to bully the international community into negotiating on Pyongyang's terms.

"They are upping the rhetorical ante in every possible way, but the international community is not reacting as it had hoped," said Cho Han-Bum, an analyst at the Korea Institute for National Unification.

Cho said the coming politburo meeting would probably seek to keep "the momentum going" through some symbolic gesture.

"I envisage a resolution that further raises the alarm, like declaring a top alert for the entire nation beyond the military, or something like that," Cho said.

Although North Korea is a past master of brinkmanship, there are concerns in South Korea and beyond that the current situation is so volatile that one accidental step could escalate into serious conflict.

Tuesday's military warning of strikes on US military bases in the Pacific prompted a response from Japan which, unlike many of the other potential targets mentioned, is well within the range of the North's current missile arsenal.

"We demand that provocative actions must not be taken. The government is on full alert," said Chief Cabinet Secretary Yoshihide Suga.

Pentagon spokesman George Little said US forces were ready to respond to "any contingency".

The US and South Korean militaries signed a new pact last week, providing for a joint military response to even low-level provocation by North Korea.

In a sign of the growing tensions along the North-South border -- one of the most heavily fortified frontiers in the world -- a South Korean guard Wednesday threw a grenade at an "unidentified object" and triggered a brief alert.

A subsequent patrol uncovered no sign of any North Korean infiltration, a defence ministry spokesman said.

For all the dire threats emanating from the North, officials in the South say they have detected no substantial troop deployment to suggest an imminent attack.

However, there are fears Pyongyang might resort to a limited artillery assault similar to its 2010 shelling of a South Korean border island that killed four people.