By Chris Hogg BBC News, Tokyo

Japan usually just says the bombing was "regrettable"

Shoichi Nakagawa, the policy chief of the governing party, said that the use of atomic weapons was a crime.

Mr Nakagawa has attracted controversy recently, calling for a debate on whether Japan should have nuclear arms.

He raised the possibility that North Koreans might try to attack Japan with their own nuclear weapons.

Speaking in Nagasaki over the weekend, Mr Nakagawa - a right-winger - said that atomic bombings were a crime. The American decision to drop the atomic bomb was truly impermissible on humanitarian grounds, he said.

He repeated the comments on Monday, telling Reuters news agency: "By dropping two atomic bombs, many people, including ordinary citizens, were killed... I believe that such an act can be called a crime."

Stronger stance

After the nuclear attacks in 1945, the Japanese wartime government condemned the bombings as crimes against international law.

But later on the authorities gave up any idea of pursuing the issue of criminality.

Today the phrase the government more often uses to describe the attack is "regrettable".

Mr Nakagawa appears to be going further, saying they were impermissible on humanitarian grounds.

Analysts say that Mr Nakagawa might have made the comments to simply acknowledge the understandable concern there might be in Nagasaki about his calls for a debate about nuclear weapons, and that this was an attempt to allay those fears with a strong condemnation of the use of the atomic bomb.

"Nakagawa must have said what he said in Nagasaki with a desire to 'nuance' his repeated statements that there was nothing wrong for Japan to have a debate on itself going nuclear - and make it more acceptable," said Koichi Nakano from Sophia University.

At the same time, though, he points out that Mr Nakagawa's reference to a war crime by the Americans that was never really prosecuted is a favourite theme of right-wingers in Japan.

The politician's comments were probably intended simply for a local audience and were not thought likely to be picked up elsewhere.

Mr Nakagawa also used his speech to warn of the threat posed by North Korea.

"There exists a country that appears likely to use them if it does not like something," he said.