WASHINGTON – Private firms conducting business in Iran before international sanctions are lifted “do so at their own peril,” US President Barack Obama said on Tuesday during a joint press conference with visiting French President François Hollande.Should businesses proceed with contracts in violation of sanctions imposed by the United States, the European Union and the UN Security Council, “we will come down on them like a ton of bricks,” Obama said from the East Room of the White House.World powers were “firm” in their commitment to their sanctions regime, which has led to a dramatic contraction of the Iranian economy, he said. But all were in agreement that new sanctions legislation during the current negotiations period could endanger prospects for a diplomatic solution to the nuclear crisis, Obama said.Republicans in the US Congress have threatened to use partisan procedural tactics to force a vote in the Senate on new conditional sanctions, skeptical that the interim deal reached in November has truly halted Iran’s march toward nuclear weapons capacity.Eluding to criticisms in Congress, and to the standard Israel’s government has set for any comprehensive accord between world powers and the Islamic Republic, Obama said he would maintain a “very high threshold” for proof from Iran that its nuclear ambitions were limited to civilian power.Questioned on the visit of a large French trade delegation to Tehran last week, Hollande said he cannot control the travel of business executives as president of the republic.“Companies just make their decisions when it comes to travel,” he said, adding that he had made “clear” to French businessmen what sanctions entail.“Sanctions will only be lifted if and when there is definite agreement,” Hollande said. “During this period of the interim agreement, they remain in force.”“Toughness” forced Iran to agree to an interim deal, freezing much of its nuclear work in exchange for $7 billion in sanctions relief, Hollande said.But there is and will remain an “Iranian problem” until the Islamic Republic renounces its nuclear program, he added.France has become a key ally to the US on matters of policy in the Middle East. Hollande pointed out that, as the US was nearing military action against Syria last September in response to President Bashar Assad’s use of chemical weapons against civilians, France was prepared to act in concert – where other allies, such as Great Britain, were not.But France now supports the diplomatic path that opened in response to the threat of military force, he said. Hollande promised to exert pressure on Assad to speedily remove all chemical weapons from Syria.Obama, who has been criticized for not using force against Assad after threatening to do so, proudly lauded the policy and said that Assad’s chemical arms stockpile had now been “completely chronicled.”Hollande was honored at the White House in the first official state visit by a French president in nearly 20 years. He was welcomed with pomp, as French and American flags draped nearly a mile of Pennsylvania Avenue. Hollande arrived to cannon fire off the Potomac River and to a full display of an honor guard.US Vice President Joseph Biden, Secretary of State John Kerry and National Security Adviser Susan Rice were in attendance at the ceremony on the South Lawn.In his opening remarks during their joint press conference, Hollande briefly mentioned US efforts at forging peace between Israel and the Palestinians, urging the parties to sign a framework for negotiations “now.”Obama, too, briefly mentioned the negotiations, saying that the EU would play a key role in the implementation of a final-status peace accord.