The North also said it would not join multilateral talks proposed by the United States, saying: "Dialogue and pressure are not compatible."
Secretary of State Colin Powell, meanwhile, urged Asia-Pacific nations at a meeting in Cambodia to support Washington's effort to form a coalition of countries to curb North Korea's nuclear ambitions.
The United States and allies including Japan and Australia have recently put their navies on alert for North Korean vessels that may be carrying illicit drugs, weapons and counterfeit money — believed to be key sources of hard currency for the impoverished North.
The communist state's official news agency KCNA on Wednesday quoted an unidentified Foreign Ministry spokesman as saying: "We will step up our efforts to strengthen our nuclear deterrent capabilities as a means of self-defense against the United States' strategy to isolate and stifle" the North.
"The Iraqi war proved that disarmament leads to a war," Rodong said in a commentary carried by KNCA. "Therefore, it is quite clear that the DPRK can never accept the U.S. demand that it scrap its nuclear weapons program first," it said.
The North's official name is the Democratic People's Republic of Korea.
Earlier Wednesday, Pyongyang's main state-run newspaper Rodong Sinmun made the first ever explicit reference to the North's own "nuclear weapons program."
Previously, Northern officials had only confirmed the communist country had one in private comments to U.S. officials at meeting in Beijing earlier this year. Last week, North Korea said it had an "intention to build up a nuclear deterrent force."
U.S. officials have said North Korea is believed to have at least one or two nuclear weapons. Since April, North Korea has claimed that it had all but completed reprocessing 8,000 spent nuclear fuel rods — a process that could yield several more bombs within months.
Wednesday's comments marked an intensification of the North's often bellicose rhetoric in its standoff with Washington.
For days, North Korea has claimed that the United States and its allies were laying "international siege" to the isolated country, as a prelude for invasion. The United States says it has no plans to invade North Korea, but has not ruled out the military option.
The North Korean spokesman called the U.S. allies' naval crackdown a "violation of the armistice" that ended the 1950-53 Korean War, "a declaration of war," and "tantamount to the very act of war."
In Phnom Penh, Powell told ministers at a security forum of the Association of Southeast Asian Nations, or ASEAN, that resolving the nuclear standoff peacefully would require coordinated pressure by nations on North Korea.
"This is not a bilateral matter between the United States and North Korea," he said, according to a State Department official who attended the closed-door meeting where Powell spoke. "It affects every nation in the region that would fall under the arc of a North Korean missile."
"ASEAN's help in keeping pressure on North Korea is absolutely necessary if we are to achieve the goal that all of us seek: a diplomatic solution that leaves the peninsula, the region and the world safer," Powell said.
Washington wants talks on North Korea's nuclear ambitions to include Russia, China, South Korea and Japan. North Korea wants bilateral negotiations with Washington, but it has recently said it might consider U.S. demands for talks involving several nations, if it can also meet one-on-one with the United States.
The nuclear dispute flared in October when U.S. officials said North Korea admitted it had a clandestine nuclear program in violation of a 1994 agreement with Washington.
The United States and its allies suspended fuel shipments promised under the 1994 deal, and Pyongyang retaliated by expelling U.N. monitors, restarting frozen facilities capable of making nuclear bombs and withdrawing from the Nuclear Nonproliferation Treaty.
North Korea claims its moves are aimed at self-defense, prompted by the Bush administration listing it as part of an "axis of evil," erecting a missile defense aimed at stopping North Korean launched, and adopting a policy of preemptive war.
According to published reports, the North also wants nuclear weapons so it can whittle down its large and very expensive conventional force.
But many observers believe the economically isolated North, where famine is a perennial risk, is merely trying to squeeze more aid out of Western donors.