President Barack Obama warned Tuesday that America would take tough action against firms violating sanctions against Iran, after a French commercial mission raised hackles in Washington.
France and the United States work closely together to maintain pressure on Iran to drop its alleged nuclear weapons program and, if anything, Paris has adopted the tougher diplomatic stance.
But last month a French commercial delegation visited Tehran to study investment opportunities, threatening to this united front before French President Francois Hollande's state visit to Washington.
On Tuesday, the two leaders appeared alongside each other at the White House to field questions from reporters and to insist on the strength of their coordinated approach to the Iranian stand-off.
Obama had tough words for anyone hoping the now looser embargo -- Tehran's reward for signing an interim deal to allow tighter controls on its nuclear program -- would leave an open door.
"Businesses may be exploring: 'Are there some possibilities to get in sooner rather than later if and when there is an actual agreement to be had?'," Obama said, standing alongside Hollande.
"But I can tell you that they do so at their own peril right now. Because we will come down on them like a ton of bricks."
Hollande said he does not control French corporations but made it clear that sanctions on Iran would not be dismantled until a final deal on Iran's nuclear program had been reached.
"So companies just make their decisions when it comes to traveling, but I certainly let them know that sanctions were in force and would remain in force," Hollande said.
"Sanctions will only be lifted if and when there is definite agreement," Hollande said, insisting that Iran must have "renounced the nuclear weapon, fully and comprehensively."
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The 116-strong French delegation, with representatives from major companies like Total, Lafarge and Peugeot, was the largest of its kind from Europe since November's landmark nuclear deal.
Under that agreement, Iran gained limited relief from crippling US and EU sanctions, but must halt some of its nuclear fuel enrichment and allow the UN nuclear watchdog access to its plants.
The sanctions have had a devastating effect on the Iranian economy, but it remains a major oil exporter and a huge consumer market with a relatively skilled workforce and huge commercial potential.
Many French firms have longstanding commercial arrangements with Iran despite political tensions and Paris' steadfast opposition to its nuclear drive, which the Islamic regime insists is peaceful.
Obama has welcomed the interim nuclear deal as a victory for his muscular diplomacy, but he faces domestic pressure not to move too quickly on the easing of sanctions.
Some US lawmakers are even pushing for stronger measures, despite warnings from the White House that this could scupper the negotiations and lead to a military confrontation with Iran.
Israel, a US ally and itself an undeclared nuclear power, has not ruled out strikes to derail a program it sees as an attempt to acquire atomic weapons and tip the balance of power in the Middle East.
"We don't want new sanctions, because the ones we have in place are already squeezing Iran and brought them to the table," Obama told the news conference.
"But we also want to send a message to the Iranians ... that the sanctions regime not only will stay in place, but will likely be tightened in the event these talks fail," he said.
Hollande is on the second day of a state visit to Washington, during which France has been hailed as one of America's closest allies,