Japan asked the United States in 1965 to be ready to attack China with nuclear weapons if the two Asian powers went to war, newly declassified documents said on Monday.
Japan, the only nation to have suffered atomic attack, has long campaigned to abolish nuclear weapons -- principles that led former prime minister Eisaku Sato to win the Nobel Peace Prize in 1974.
But the foreign ministry declassified documents showing Sato sought a US nuclear strike on China in the event of a war between the two countries.
According to the diplomatic papers, Sato told then US defence secretary Robert McNamara at a 1965 meeting in Washington: "We expect the United States to retaliate immediately using nuclear weapons" in a war.
McNamara, best known as an architect of the Vietnam War, was quoted as replying only that the United States had the technical capability to deploy nuclear weapons overseas.
Sato also said that he would let the United States use Japanese waters, although not its territory, to transport nuclear weapons in the event of a war between Japan and China.
China, then a year away from launching its "Cultural Revolution," was a major cause of concern in the 1960s for World War II rival Japan. Neither Japan or the United States had diplomatic ties with communist China until the 1970s.
Asked Monday about Sato's remarks, Chief Cabinet Secretary Takeo Kawamura, the Japanese government's spokesman, defended the then premier by noting that China had just carried out nuclear tests.
"We can reach a conclusion that no nuclear weapons have been brought to Japan," Kawamura, a member of Sato's long-dominant Liberal Democratic Party, told a news conference.
Sato's three-point non-nuclear policy -- that Japan will not produce, possess or allow the entry of nuclear weapons -- is "determined and steadfast," Kawamura said.
"Washington has understood this," he said.
Sato led Japan from 1964 to 1972, making him the country's longest-serving prime minister.
The United States dropped atom bombs in 1945 on the Japanese cities of Hiroshima and Nagasaki, killing more than 210,000 people.
But the United States is now the main ally of officially pacifist Japan and stations more than 40,000 troops on its soil.