Prime Minister Shinzo Abe, a conservative who took office in September, has repeatedly said he will not consider developing nuclear weapons, a long-time taboo in the only nation to have suffered atomic attack.

But Japan's four opposition parties decided in a meeting to visit Abe and urge him to sack Aso.

"The four opposition parties agreed we will demand the foreign minister's resignation for recommending discussion on having nuclear weapons," said Hirotaka Otaki, an official of the main opposition Democratic Party of Japan.

"We can't help but suspecting that Aso is talking on behalf of the prime minister, who cannot admit his real beliefs that Japan should become a nuclear power," he said.

Aso and Shoichi Nakagawa, a top policy aide to the premier, have both called for a frank debate on going nuclear in light of communist neighbor North Korea, which tested an atom bomb on October 9.

But both have stopped short of openly calling for an end to Japan's 1967 "three principles" of refusing the possession, production and presence of nuclear weapons on its soil.

Abe in parliament defended himself against opposition criticism, saying, "No one in my cabinet is against the non-nuclear principles."

"You have to differentiate between the issue of having nuclear weapons and the issue of how to contain the threat of nuclear attacks," Abe said.

But main opposition leader Ichiro Ozawa said that Abe needed to muzzle his aides on the issue.

"If the government is to keep to its three-point non-nuclear principles, cabinet members should refrain from comments that can cause misunderstanding both within the country and abroad," Ozawa said.

"As the only nation which suffered attacks by atomic bombs, the Japanese government should deal with this problem carefully," he said.

US nuclear bombs obliterated the Japanese cities of Hiroshima and Nagasaki in the closing days of World War II, killing more than 210,000 people.

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