"We have to prepare for the worst, and the worst is war," he said in an interview broadcast on television and radio.
"We must negotiate right to the end," with Iran, he said, but underlined that if Tehran possessed an atomic weapon, it would represent "a real danger for the whole world."
Kouchner added France had advised its large companies not to respond to tenders in Iran and repeated a call for greater pressure on Tehran over its nuclear program.
Kouchner said the companies that had been contacted were free to decide what to do.
"We have already asked a certain number of our large companies to not respond to tenders, and it is a way of signaling that we are serious," Kouchner said.
"We are not banning French companies from submitting. We have advised them not to. These are private companies. But I think that it has been heard and we are not the only ones to have done this."
AFP and Reuters contributed to the report