In a joint letter to leader Kim Jong-Il to congratulate him on his 65th birthday, state media said the communist country's party, government, military and people vowed to follow Kim's guidance to build a "powerful socialist state."

"The whole People's Army soldiers and the people will maintain war preparedness to the full, to deal with US imperialists' manoeuvres for agression," said the letter, according to the Korean Central News Agency.

"If the enemies dare ignite the fire of war, we will mobilise all our powerful combat potential, built up through the maelstrom of Songun (army-first) revolution, mercilessly crush the enemies and achieve the historic task of national unification," it said.

Such rhetoric is not unusual in a country which since its founding has regarded Washington as its principal foe.

North Korea, the United States and four other nations agreed in Beijing Tuesday on disabling the North's nuclear programme in exchange for aid and diplomatic benefits.