"I honestly don't believe (a nuclear weapon) will give any immunity to Iran," Kouchner said in an interview conducted in English with Israel's Haaretz newspaper during a two-day visit to the region.
"First, because you will eat them before. And this is the danger. Israel has always said it will not wait for the bomb to be ready. I think that (the Iranians) know. Everyone knows."
Kouchner said he hoped tough diplomacy and sanctions would persuade Iran to halt its uranium enrichment programme, which Israel and many Western countries believe is aimed at developing nuclear weapons.
"Iran with an atomic bomb is unacceptable at all... Talking, talking talking, and offering dialogue, sanctions, sanctions, sanctions. Is the alternative to bomb first -- I think not."
Iran has always insisted its nuclear drive is entirely peaceful. Israel is widely regarded as the only nuclear armed state in the Middle East but it has never confirmed or denied having an arsenal.
Kouchner is set to meet outgoing Prime Minister Ehud Olmert and Foreign Minister Tzipi Livni -- who is currently struggling to form a new coalition government to succeed Olmert -- on Sunday.
France currently holds the rotating presidency of the European Union, which has been sponsoring Israeli-Palestinian peace talks as part of the Middle East Quartet, which also includes the United States, the United Nations, and Russia.
On Saturday Kouchner toured the West Bank town of Jenin, the focus of a months-old Palestinian security crackdown that has been praised by Israel and the United States -- and met Palestinian president Mahmud Abbas.
Kouchner hailed the peace talks which were formally relaunched last November but said they were unlikely to meet their stated goal of a comprehensive agreement by the end of the year.