In an interview with Haaretz published on Sunday, Kouchner, who is on a visit to Israel, said he agreed with expert estimates that Iran needed from two to four years to create a nuclear bomb.
"I think that you are well informed, and so are the Americans and so are we. It has always been the case between two and four years. But to make what? One bomb," Haaretz quoted Kouchner as saying.
Kouchner said that even one bomb would not make Iran immune to attack.
"I honestly don't believe that it will give any immunity to Iran. First, because you will eat them before. And this is the danger. Because Israel has always said that it will not wait for the bomb to be ready," he said.
Kouchner also urged to give international diplomacy more time to solve the Iranian nuclear issue.
"Iran with an atomic bomb is unacceptable at all...But how can it be prevented? Talking, talking, talking, and offering dialogue, sanctions, sanctions, sanctions," he said.
The UN Security Council unanimously approved a resolution on September 27, reiterating demands for Iran to halt uranium enrichment. The resolution did not however impose any new sanctions against the Islamic Republic.
Iran says its nuclear program is aimed purely at electricity production, and has pledged to continue uranium enrichment regardless of international sanctions.