From the 1950s Pentagon to today's Obama administration, the United States has repeatedly pondered, planned and threatened use of nuclear weapons against North Korea, according to declassified and other U.S. government documents released in this 60th-anniversary year of the Korean War.

Air Force bombers flew nuclear rehearsal runs over North Korea's capital during the war. The U.S. military services later vied for the lead role in any "atomic delivery" over North Korea. In the late 1960s, nuclear-armed U.S. warplanes stood by in South Korea on 15-minute alert to strike the north.

Just this past April, issuing a U.S. Nuclear Posture Review, Defense Secretary Robert Gates said "all options are on the table" for dealing with Pyongyang — meaning U.S. nuclear strikes were not ruled out.

The stream of new revelations about U.S. nuclear planning further fills in a picture of what North Korea calls "the increasing nuclear threat of the U.S.," which it cites as the reason it developed its own atom-bomb program — as a deterrent.

"This is the lesson we have drawn," North Korea's vice foreign minister, Pak Kil Yon, told the U.N. General Assembly in New York on Sept. 29.

The new information is contained in Korean War documents released by the CIA to mark this June's anniversary of the start of the conflict; another declassified package obtained by Washington's private National Security Archive research group under the Freedom of Information Act; and additional documents, also once top-secret and found at the U.S. National Archives, provided to The Associated Press by intelligence historian and author Matthew Aid.

Expert observers are speculating that North Korea, which conducted nuclear tests in 2006 and 2009, may soon stage another. Pyongyang's program "has now reached an extremely dangerous level," Kim Tae-hyo, a South Korean government security adviser, said in comments published Wednesday in Seoul.

In a report on global nuclear threats, analysts at Washington's Stimson Center identify six overt warnings by high-ranking American officials since 1976 that the U.S. would resort to nuclear weapons against North Korea if warranted. But U.S. threats go back more than a half-century, to long before North Korea split its first atom.

In mid-August 1950, just seven weeks after North Korea invaded South Korea and five years after two U.S. atomic bombs killed at least 220,000 Japanese in Hiroshima and Nagasaki, U.S. nuclear weapons were first assigned to the new war theater, according to a declassified Army planning document obtained by the AP.

Retreating U.S. and South Korean troops were then desperately clinging to a last-ditch salient in Korea's southeast, from which they soon broke out in a counteroffensive that took them into North Korea.

That November, after Chinese troops joined in defending North Korea, then-President Harry Truman rattled the nuclear saber at a Washington news conference, saying, "There has always been active consideration of its use."

Regional U.S. commander Gen. Douglas MacArthur, in interviews published posthumously, said he had a plan at the time to drop 30 to 50 atom bombs across the northern neck of the Korean peninsula, to block further Chinese intervention.

Based on previously declassified documents, however, historians believe the U.S. came closest to unleashing its atomic arsenal against North Korea in April 1951, on the eve of an expected Chinese offensive.

With Truman's signoff, the Joint Chiefs of Staff ordered A-bomb retaliation if large numbers of fresh Chinese troops entered the fight. In the end, the U.S. military repelled the Chinese push and the weapons were never used. But Pentagon planners retained the option.

In September and October 1951, Air Force B-29 bombers conducted simulated atomic-bombing runs against Pyongyang, dropping dummy weapons on the North Korean capital, according to a newly obtained Army planning document corroborating earlier disclosures.

By early 1953, the U.S., frustrated by stalled armistice talks, pondered launching a new offensive against the north Koreans and Chinese. The Pentagon's Air Staff recommended using A-bombs to achieve victory "in the shortest space of time," according to a Feb. 20, 1953, memo from the Air Force director of plans, Maj. Gen. Robert Lee.

Added a top-secret CIA Special Estimate, "The Communists would recognize the employment of these weapons as indicative of Western determination to carry the Korean war to a successful conclusion."

Then, in a series of memos in May, June and July 1953, Air Force generals reported progress in planning an "atomic offensive" to "destroy effective Communist military power in Korea" if the armistice talks broke down completely.

On July 27, 1953, an armistice was signed. Then-President Dwight D. Eisenhower would later credit the nuclear threat — conveyed through back channels to Beijing — for pressuring the Chinese into an agreement.

Even without nuclear weapons, three years of U.S. conventional bombing had devastated North Korea, killing hundreds of thousands of civilians.

The nuclear planning didn't stop with the fighting. On Aug. 20, 1953, declassified documents show, the Strategic Air Command sent Air Force headquarters a plan for "an air atomic offensive against China, Manchuria and North Korea" if the communists resumed hostilities. "OpPlan 8-53" called for use of "large numbers of atomic weapons."

The post-armistice respite, meanwhile, stirred up inter-service rivalries.

Air Force commanders asked for more nuclear-capable F-84G warplanes in the Korea theater "to offset the Navy's greater and more immediate atomic delivery capability," the declassified documents show. But one colonel warned against arousing "the Army-Navy suspicion that the Air Force is trying to steal the atomic bomb act" in Korea planning.

By the late 1950s, all the services shared in an "era of relative atomic plenty," as an Air Force memo called it. The number of nuclear warheads in South Korea and nearby Okinawa — in artillery shells, short-range missiles, gravity bombs and other weapons — peaked at about 2,600 in 1967, civilian researchers would later determine.

In 1969, after the North Koreans shot down a U.S. reconnaissance plane over the Sea of Japan, then-President Richard Nixon's lieutenants had these nuclear tools at hand for laying out retaliatory options.

"USAF tactical fighters armed with nuclear weapons are on 15-minute alert in ROK (Republic of Korea) to strike airfields in North Korea," said the contingency plan Defense Secretary Melvin Laird sent to White House national security chief Henry Kissinger, a document obtained by the National Security Archive.

In the end, Nixon decided against military retaliation. The Pentagon had noted that the reaction of China and the Soviet Union, both nuclear-armed, was unpredictable.

In 1975, in response to a perceived North Korean threat of renewed war, President Gerald Ford's defense secretary, James Schlesinger, openly confirmed the presence of U.S. nuclear weapons in South Korea for the first time, and warned North Korea, "I do not think it would be wise to test (U.S.) reactions."

President Jimmy Carter's administration later scaled back the Korea-based arsenal, and its complete withdrawal was announced in 1991, although the North Koreans at times accuse the U.S. of maintaining a secret nuclear stockpile in the south.

Korea specialists generally accept Pyongyang's stated rationale that it sought its own bomb for defensive reasons — "as a response to the U.S. deployment of tactical nuclear weapons in South Korea," says author Selig Harrison.

Yoshiki Mine of Japan's Canon Institute for Global Studies, who as a diplomat dealt with both disarmament and North Korea, said the northern regime feels its existence as a nation is threatened.

The U.S. nuclear option "does give the North Koreans an excuse to develop, acquire and own nuclear weapons," Mine told the AP. "They have indicated many times that as long as this basic security is not secured, they would not abandon nuclear weapons."