Mr. Rouhani said that a reconstituted nuclear program would be “far more advanced,” a veiled threat that the country could start enriching uranium up to the level of 20 percent, a step toward building a nuclear weapon. Such enrichment activities were a major concern before 2015 when Tehran signed a landmark agreement with the United States and other world powers that lifted crippling economic sanctions in return for severe limits on Iran’s nuclear activities.
President Trump has repeatedly called the nuclear deal a “disaster,” said that he believes the Iranians are violating its terms, and twice called for reviews, in hopes of finding reasons to kill it.
The other parties to the nuclear deal — Britain, China, France, Germany, and Russia — disagree with Mr. Trump, saying that Iran is not in violation of the pact, which is overseen by the United Nations nuclear watchdog, the International Atomic Energy Agency.
New sanctions approved by Congress this month penalized Iran for its ballistic missile program and other activities in the region. Iran says that it has the right to conduct missile tests and that it has fully complied with the provisions of the nuclear agreement.
While Mr. Rouhani was re-elected this year, he has faced increasing pressure from hard-liners, who have said all along that the United States is not to be trusted and will never deliver on its promises. The economic benefits Mr. Rouhani promised from the signing of the nuclear deal have not been completely realized, largely because of unilateral American sanctions that have deterred much foreign investment.
Mr. Trump’s threats to withdraw from the nuclear agreement have added weight to the hard-liners’ arguments, putting Mr. Rouhani ever more on the defensive and weakening him politically. On Monday, for example, conservatives were able to tighten their grip on the Expediency Council, one of Iran’s most influential oversight bodies.
Mr. Rouhani warned the Trump administration that Iran could react quickly if further sanctions were confirmed. “The new U.S. officials should know that the failed experience of sanctions and coercion compelled their previous governments to eventually come to the negotiation table,” Mr. Rouhani said. “If they want to try those experiences again, Iran will definitely revert to a far more advanced situation than it had before the negotiations, not in a matter of weeks or months but in a matter of days or hours,” he told lawmakers.
Mr. Rouhani also noted that Mr. Trump had pulled out of several international treaties or was threatening to do so. “It is the U.S. government, especially the current Trump administration, that is ignoring international treaties,” he said, “showing to the world and its allies that the U.S. is neither a good ally nor a trustworthy negotiating partner.”
Several other Iranian officials have recently threatened to restart industrial-scale uranium enrichment. Ali Akbar Salehi, president of the Atomic Energy Organization of Iran, said the country could go up to 20 percent enrichment to “surprise the Americans and their supporters.”
Such enrichment by Iran would be a violation of the nuclear agreement, known officially as the Joint Comprehensive Plan of Action, which requires that Iran limit uranium enrichment to 3.67 percent, the level typical for fueling nuclear power reactors.
Analysts say Iran probably would be extremely careful to avoid exceeding the enrichment limit, so as to not alienate the other countries that are part of the nuclear accord.
The European Union, which strongly supported global sanctions against Iran under President Barack Obama, has started to invest in the country since the deal was signed, and it is not likely to support new penalties. China and Russia have been partners to the Iranians for many years.
Mr. Rouhani’s warning was sharply criticized by the Trump administration. Nikki R. Haley, the American ambassador to the United Nations, said in a statement that the warning amounted to an Iranian attempt at blackmail.
“Iran cannot be allowed to use the nuclear deal to hold the world hostage,” Ms. Haley said in the statement, titled “Ambassador Haley on Iran’s Threats to Quit the JCPOA.”
The new United States sanctions on Iran, she said, were not a violation of the nuclear deal but part of an effort to “hold Iran responsible for its missile launches, support for terrorism, disregard for human rights, and violations of U.N. Security Council resolutions.”