Former British premier Tony Blair warned in an interview Wednesday that the international community may have "no alternative" to taking military action against Iran if it develops a nuclear weapon.

"I am saying that I think it is wholly unacceptable for Iran to have a nuclear weapons capability and I think we have got to be prepared to confront them, if necessary militarily," he said in extracts pre-released by the BBC from an interview to publicise his memoirs.

"I think there is no alternative to that if they continue to develop nuclear weapons. They need to get that message loud and clear."

Blair expanded on the point in his book, which is being published Wednesday, saying that while a nuclear-armed Iran would pose a threat to the West, the real danger would come with its ability to boost Islamic extremism.

"Let us be frank: Iran is a far more immediate threat to its Arab neighbours than it is to America" or Britain, he wrote in the memoir, which recounts his experiences during his decade as prime minister from 1997 to 2007.

"That's why Iran matters. Iran with a nuclear bomb would mean others in the region acquiring the same capability; it would dramatically alter the balance of power in the region, but also within Islam."

Western powers and Israel accuse Iran of trying to build a nuclear arsenal.

Iran, which on Saturday began loading fuel into its Russian-built first nuclear power plant, denies the allegations, saying its programme is for civil energy purposes only.