Japan's government Monday denied a report that Tokyo had secretly agreed with Washington during the cold war to allow US warships carrying nuclear weapons to make port calls in the country.

"Such speculation has gone on for decades, but the government of Japan has said there is no such secret agreement," said a foreign ministry official.

The Mainichi Shimbun national daily on Monday published a report saying that such a secret document existed, quoting Ryohei Murata, a former vice foreign minister who served in the late 1980s.

The issue is sensitive in Japan, the only country to have suffered nuclear weapon attacks when bombed by the United States towards the end of World War II.

Since then it has heavily relied on its alliance with the United States for its defence, including nuclear deterrence.

Japan in 1968 adopted a "three non-nuclear principles" policy of not possessing or producing nuclear weapons or allowing them on its territory, and it regularly speaks out in favour of a nuclear-weapons-free world.

The newspaper said the accord was reached when Japan and the United States revised their security treaty in 1960 to give more equal footing to Japan, which was rebuilding itself from the devastation of World War II.

Murata -- who served as vice minister for about two years from 1987 -- reportedly said that when he took his post, his predecessor gave him a document in an envelope, asking him to "brief the minister on its content."

"I had heard that there was a secret agreement, but I saw the Japanese side's paper for the first time when I became vice minister," Murata said according to the report.

When Washington and Tokyo revised their treaty, they agreed that the United States must consult with Japan before making any major changes to its deployment of equipment or troops in Japan.

The foreign ministry official, who declined to be named, told AFP: "Such a prior consultation has never taken place. Therefore there has been no case of us letting nuclear weapons in."

Foreign Minister Hirofumi Nakasone last week denied a media report that the government had also demarcated its territorial waters so as to avoid disputes over the movement of US warships carrying nuclear weapons.