France has urged its EU partners to expand economic sanctions against Iran to end its "nuclear defiance".
Bernard Kouchner has said the world should prepare for a war with Iran
French Foreign Minister Bernard Kouchner said Iran was close to mastering uranium enrichment which he said was a de facto military capacity.
Paris is likely to be backed by Britain, but Germany and Italy oppose tougher sanctions, correspondents say.
Iran denies Western accusations that it is seeking a nuclear weapon, saying its nuclear programme is for civilian use.
The UN Security Council has imposed sanctions on Iran because of its refusal to suspend all uranium enrichment activity.
Last month, the five permanent council members and Germany delayed a decision on further sanctions, saying they would wait until they saw UN and EU reports before drafting a resolution.
'Dialogue and firmness'
In a letter to his EU counterparts, Mr Kouchner said: "In parallel to negotiations on a new Security Council resolution, which must be pursued, I propose that we start right away exploring together the possibility of new European measures."
"It is vital that the EU today show its determination and set an example once again by taking the initiative of firm new measures", the letter said.
Iran denies its nuclear work is intended to develop weapons
It added that such a move from the EU - Iran's main trade partner - would increase pressure on Tehran.
Mr Kouchner also warned that the 27-member bloc must act without delay: "Time is working against us, since each day Iran moves closer to mastering the technology of enrichment, which means a de facto military nuclear capacity."
The letter also reaffirmed that the French approach on the issue was "a combination of dialogue and firmness".
Mr Kouchner last month sparked controversy by saying the world should prepare for a war with Iran - a comment that he subsequently qualified.
On Wednesday, Tehran summoned France's charge d'affaires in Tehran to protest about his comments.
France has toughened its position towards Iran since the election of President Nicolas Sarkozy earlier this year.