The United States studied a plan for a nuclear strike on North Korea in 1969 but advisers to then-president Richard Nixon concluded it was best to remain calm, declassified documents showed Wednesday.

The documents, obtained by the National Security Archive at George Washington University, foreshadow present-day US frustration on how to handle Pyongyang following its nuclear tests and the sinking of a South Korean ship.

In 1969, North Korea shot down a US spy aircraft over the Sea of Japan (East Sea), killing the 31 personnel on board.

Despite US outrage, the new Nixon administration chose not to retaliate other than to order a continuation of flights and go ahead with naval exercises.

The documents, released after requests under the Freedom of Information Act, showed that the administration nonetheless charted out a series of options that included conventional and nuclear attacks.

In one contingency plan codenamed "Freedom Drop," the United States would use tactical nuclear weapons to destroy military command centers, airfields and naval bases in North Korea.

Civilian casualties "would range from approximately 100 to several thousand," said a classified memorandum by then-defense secretary Melvin Laird prepared for Henry Kissinger, who was Nixon's national security adviser.

There is no indication that the administration seriously considered a nuclear strike. The document stated that the United States could use one nuclear option if North Korea launched an air attack on the South.

In a document recounting a White House meeting, Kissinger is quoted as saying that his initial reactions to the spy plane incident "were probably naive" and that it was most crucial to prevent a "counter blow" from Pyongyang.

"The need is to look determined and, if the object is to prevent counter-responses, the action taken should be (a) powerful blow," Kissinger said.

"If a similar situation were to arise today, (Nixon) would probably either do nothing or select an option toward the extreme of the range of possibilities," Kissinger said.

The United States this week marks the 60th anniversary of the Korean War, which ended with an armistice and the peninsula still divided.

Since the conflict, the United States has repeatedly -- and sometimes begrudgingly -- relied on carrot-and-stick diplomacy with North Korea, concluding it was the only realistic option.