The salvo in Pyongyang's main Rodong Sinmun newspaper was the North's latest reaction to last week's summit between President Barack Obama and South Korean President Lee Myung-bak. The allies issued a joint statement committing the US to defend the South with nuclear weapons.
It also came as an American destroyer trailed a North Korean ship suspected of shipping weapons in violation of a UN resolution punishing Pyongyang's May 25 nuclear test, and as anticipation mounted that the North might test-fire short- or mid-range missiles. The North's newspaper claimed in a lengthy commentary that the "nuclear umbrella" commitment made it more likely for the US to mount a nuclear attack on the communist North, and only "provides us with a stronger justification to have nuclear deterrent."
It also amounts to "asking for the calamitous situation of having a fire shower of nuclear retaliation all over South Korea" in case of a conflict, the paper said. "It is as clear as daylight that South Korea cannot survive under that nuclear umbrella." In a separate editorial marking the 1950 outbreak of the Korean War, the Rodong said the North "will never give up nuclear deterrent no matter what others say and will further strengthen it" as long as Washington remains hostile toward it.
The brutal fighting ended after three years in a truce in 1953, not a peace treaty, leaving the peninsula divided and in a state of war. The US has 28,500 troops in South Korea to protect against an outbreak of hostilities.
North Korea has long claimed that the US is plotting to invade it and has used the claim to justify its development of nuclear weapons. On Wednesday, Pyongyang accused Washington of seeking to "provoke a second Korean War," saying it will "wipe out the aggressors on the globe once and for all." The US has repeatedly said it has no intention of attacking the North.
The UN resolution seeks to clamp down on North Korea's trading of banned arms and weapons-related material by requiring UN member states to request inspections of ships carrying suspected cargo.
The US has been seeking to get key nations to enforce the sanctions aggressively. Secretary of State Hillary Rodham Clinton called the foreign ministers of Russia and China to discuss efforts to enforce UN punishments of North Korea for its nuclear test, State Department spokesman Ian Kelly said.
The North's ship, Kang Nam, is believed to be the first North Korean vessel to be tracked under the resolution. It left the North Korean port of Nampo a week ago and is believed bound for Myanmar, South Korean and US officials said.
Myanmar state television played down the reports of a possible weapons shipment Wednesday evening, saying another North Korean vessel was expected to pick up a load of rice but that the government had no information about the Kang Nam.
A senior US defense official said Wednesday that the ship had already cleared the Taiwan Strait.
He said he didn't know how much range the Kang Nam has - that is, whether or when it may need to stop in some port to refuel - but that the Kang Nam has in the past stopped in Hong Kong's port.
Another US defense official said he tended to doubt reports that the Kang Nam was carrying nuclear-related equipment, saying the information officials have received seems to indicate the cargo is conventional munitions.
The US officials spoke on condition of anonymity because they were discussing intelligence.
The US and its allies have not decided whether to contact and request inspection of the ship, Pentagon press secretary Geoff Morrell said Wednesday.
"That's a decision that will have to be made at some point, and not necessarily just by us or this government," he said at a news conference. "I think we will likely take (the decision) collectively with our allies and partners." He said he didn't believe a decision would come soon.
North Korea has said it would consider interception of its ships a declaration of war.
Reports about possible missile launches from the North highlighted the state of tension on the Korean peninsula.
The North has designated a no-sail zone off its east coast from June 25 to July 10 for military drills.
A senior South Korean government official said the ban is believed connected to North Korean plans to fire short- or mid-range missiles. He spoke on condition of anonymity, citing department policy.
Yonhap reported that the North may fire a Scud missile with a range of up to 500 kilometres or a short-range ground-to-ship missile with a range of 160 kilometres during the no-sail period.
US defense and counterproliferation officials in Washington said they also expected the North to launch short- to medium-range missiles. The officials spoke on condition of anonymity to discuss sensitive intelligence.
North Korea had warned previously it would fire a long-range missile as a response to UN Security Council condemnation of an April rocket launch seen as a cover for its ballistic missile technology.