North Korea issued an ominous new threat Monday in its campaign against South Korean President Lee Myung-bak, vowing to carry out a special military attack that would reduce parts of Seoul to ash “in three or four minutes . . . by unprecedented peculiar means and methods of our own style.”

The warning came as government officials in Seoul and Washington are watching for clues about the emerging strategy of new and untested leader Kim Jong Eun, who came to power four months ago after the death of his father, Kim Jong Il. This month, the young leader broke a weapons-testing moratorium by firing a long-range rocket, the first suggestion that he will try to consolidate power domestically by showing off his country’s military might and keeping the outside world on edge.

The North’s state-run news agency said the “targets are the Lee Myung-bak group of traitors, the arch criminals, and the group of rat-like elements including conservative media destroying the mainstay of the fair public opinion.”

The statement, attributed to a “special operation action group” of the North Korean military, provided no details about how the attack might be carried out. But analysts said the statement differed from Pyongyang’s usual calls for sacred war and fiery revenge — background noise on the peninsula — because it laid out the scenario for a specific, targeted military strike.

“I suppose their threat is very concrete,” said Park Hyeong-jung, a North Korea researcher at Seoul’s Korea Institute for National Unification. “They said they will do it very soon, and this is unusual.”

The North has spent years criticizing Lee, a conservative who ended the generous aid policies of his predecessors, but it has taken sharper aim in the past week. Thousands of Pyongyang residents and army personnel met Friday in Kim Il Sung Square, pledging during a rally to “wipe out” Lee and his allies.

The North also revised its state-run news agency Web site, creating a banner that cycles through anti-Lee messages such as “Let Us Cut Off Windpipes of the Lee Myung-bak-led Swarm of Rats!” Recent articles also have called him “human scum” and an “underwit with 2MB of knowledge.”

“It is, therefore, a tragedy and shame for the nation to see Lee still alive,” one story said.

In recent days, Lee has publicly estimated the price ($850 million) of the North’s failed rocket launch on April 13 and said the money could have been used for several million tons of corn. He also suggested that Kim privatize the impoverished nation’s agricultural land to stoke the economy.

On Monday, experts in Seoul speculated that the North may be planning a cyberattack or another military provocation, similar to its island-shelling or its torpedoing of a warship in 2010. But Seoul’s Defense Ministry observed no immediate change in the North’s military movements, according to the Associated Press.

Government officials in Seoul also fear that the North is preparing for another underground nuclear test, citing excavation activity visible on recent satellite images.