Obama has made clear he plans a new approach to dealing with Iran and its nuclear program, including direct talks if needed, a break from the outgoing administration of President George W. Bush, which pursued an isolation strategy.
Kouchner, whose country holds the rotating presidency of the European Union, said he welcomed Obama's plans but urged him to be careful when dealing with the Iranians, who have been negotiating with major powers for years.
"We have negotiated at great length. People came to France, we sent people to Iran, we met them and unfortunately this dialogue produced nothing. And so, one must be careful," Kouchner told TV5 television and RFI radio.
Major powers have twice offered Iran a package of incentives to convince it to abandon uranium enrichment, a sensitive technology that can produce fuel for power plants or potentially nuclear weapons, but Tehran has rejected the advances.
The West says Iran is seeking to develop the atom bomb under the cover of a civilian nuclear program, but Iran says it only wants to master nuclear technology to generate electricity.
Unity of powers
Kouchner said his concern was that direct talks between Washington and Tehran could hurt the unity of the major powers who presented the offers to Iran and have imposed sanctions against it - France, Britain, Germany, the United States, Russia and China.
"We must warn ... against any form of dialogue that would jeopardize the unity not of the Western side but of the whole of the P5, that is the five (permanent UN) Security Council members plus Germany," he said.
Kouchner, a former aid worker poached from the political left by President Nicolas Sarkozy, also expressed doubts about Obama's plan to send more troops to Afghanistan, saying the solution there would be political, not military.
"I do not think that is the solution, except in certain places, with a specific task," Kouchner said, emphasizing the importance of being able to one day hand over responsibility for the security situation to Afghan forces.
Asked, however, if more French and European troops were needed in Afghanistan, a day after a French soldier was killed by a mine there, Kouchner said: "That is not an issue today. We will see."