President Obama warned Tuesday that the United States would come down “like a ton of bricks” on companies that looked to game the sanctions regime in Iran.

The comments from Obama during a joint press conference with French President François Hollande represent the starkest warning he has given so far that businesses should be wary of entering Iran until the administration reaches a final deal on its nuclear program.

They were also notable given his White House guest; a group of 100 executives from France’s biggest firms visited Iran earlier this month in a trip organized by the country’s employers’ association.

Obama said it might be appropriate for companies to look into the possibility of doing business in Iran, but that they did so at their own risk.

“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, “But I can tell you that they do so at their own peril right now, because will come down on them like a ton of bricks with respect to the sanctions that we control, and we expect full compliance.”

The French delegation that visited Iran included representatives from the country's energy, engineering, telecom and information technology industries, and followed similar delegations from countries including the Netherlands, Germany, Italy and South Korea, The Associated Press reported.

French automakers have been particularly hard-hit by the sanctions. Peugeot, Europe's second-largest automaker, sold nearly half a million cars annually in Iran, while Renault sold more than 100,000 per year.

The flood of international interest in Iran has come after it agreed to an interim nuclear deal with the United States and other nations. Under the deal, Iran will freeze its nuclear weapons program in exchange for temporary relief from some sanctions. Negotiators hope to strike a permanent deal to end the country's weapons program within the next six months.

Lawmakers on Capitol Hill and members of the Obama administration have warned the overtures by business could undermine negotiations.

Sen. Jeanne Shaheen (D-N.H.) in a letter to Obama last week said the French visits risked “undermining the international sanctions regime at precisely the wrong time.”

Secretary of State John Kerry told CNN that he had put French companies “on notice” that they would be sanctioned if they attempted to contravene rules prohibiting business deals with Iran.

And U.S. Undersecretary of State for Political Affairs Wendy Sherman told lawmakers that Kerry had warned French Foreign Minister Laurent Fabius that the visit could undermine negotiations.

“Tehran is not open for business because our sanctions relief is quite temporary, quite limited and quite targeted,” Sherman said.

In the press conference Tuesday, Hollande said that as president, he couldn't dictate what private corporations did.

“Companies make their decisions when it comes to traveling,” Hollande said. “But I certainly let them know sanctions were in force and would remain in force.”

He emphasized that “no commercial agreement could be signed” until a deal was in place, and French companies were “very much aware” of that fact.

“As far as sanctions are concerned, they will only be lifted if and when there is a definitive agreement,” Hollande said. 

—This story was updated at 5:49 p.m.