Iran responded Wednesday to the assassination of its top nuclear scientist by enacting a law ordering an immediate ramping up of its enrichment of uranium to levels closer to weapons-grade fuel.
The measure also requires the expulsion of international nuclear inspectors if American sanctions are not lifted by early February, posing a direct challenge to President-elect Joseph R. Biden Jr.
It was not clear whether the action was the totality of the Iranian response to the killing of the scientist, Mohsen Fakhrizadeh, whom American and Israeli intelligence agencies regarded as the guiding force of past efforts by Tehran to design a nuclear weapon, or whether more was to come. Iranian officials have vowed to avenge his killing.
The new law orders Iran’s Atomic Energy Agency to resume enriching uranium to a level of 20 percent immediately, returning Iran’s program to the maximum level that existed before the 2015 nuclear agreement reached with the Obama administration.
Uranium enriched to that level would give Iran the ability to convert its entire stockpile to bomb-grade levels within six months. Despite the lag time, the order to start the process may be seen by the Trump administration as a provocation in its waning days.
Just three weeks ago, after news of modest advances in the size of Iran’s nuclear stockpile, Mr. Trump asked his advisers about military options to stop the country from producing the fuel. He was talked out of considering an attack by Secretary of State Mike Pompeo, one of the fiercest of the Iran hawks in the administration, and the chairman of the Joint Chiefs, Gen. Mark A. Milley, among other senior officials.
The new law also sets a two-month deadline for oil and banking sanctions against Iran to be lifted before inspectors are barred, creating a potential crisis for the early days of the Biden administration. The inspections conducted by the International Atomic Energy Agency, a branch of the United Nations, have been the main public source of information about the progress of Iran’s program.
The timing seems deliberately intended to press Mr. Biden to re-enter the nuclear deal with Iran immediately upon taking office. Mr. Biden has said he would be willing to do so, at least as a starting point, if Iran once again respected the limitations of the 2015 deal.
The speaker of Iran’s Parliament, Mohammad Baqer Qalibaf, a former commander of the Islamic Revolutionary Guards Corps, said the measure was meant to send the West a message in the aftermath of the assassination that the “one-way game is over.”
Iran’s Parliament, dominated by conservatives, initially passed the law in an angry session on Tuesday in which lawmakers fumed over the killing of Mr. Fakhrizadeh. A physicist and a high-ranking official in the Defense Ministry, he was killed on Friday in a complex ambush, the details of which are still being debated.
A number of intelligence officials have said Israel was responsible for the attack, though the Israeli government has remained silent.
“The criminal enemy will not feel remorse unless we show a fierce reaction,” Mr. Qalibaf said. Lawmakers stood up in the chamber with fists in the air, chanting “death to Israel” and “death to America” as they passed the bill in a televised session.
The law was ratified Wednesday by Iran’s Guardian Council, an appointed body that oversees the elected government.
President Hassan Rouhani, whose government negotiated the 2015 Iran accord over the objection of Iranian hard-liners, had opposed the legislation, calling it counterproductive.
“The government does not agree with this legislation and considers it damaging for diplomacy,” he said Wednesday before the measure was ratified.
His government is now obliged to carry out the law, though outside experts noted that it could slow-walk the effort, citing technical challenges. And ramping production up to the levels required in the bill would require revisions in the infrastructure deep inside Iran’s main nuclear facility at Natanz — the same one that was hit by a devastating Israeli-American cyber-strike more than a decade ago.
Uranium enriched to 20 percent purity is a quick hop to bomb-grade uranium, which is roughly 90 percent pure. But getting from the current levels of 4 or 5 percent enrichment to 20 percent is a far bigger leap than the final move to bomb-grade fuel.
Iran has said its nuclear program is for peaceful purposes, though American intelligence assessed that Iran had a fully active, secret weapons program run by Mr. Fakhrizadeh until 2003. Israeli officials and some American intelligence officials believe that program has continued at a lower level; the scientist was slapped with new American sanctions in recent years.
By mandating a restoration of production and enrichment levels, the new law essentially wipes away the last of the main constraints negotiated by President Barack Obama and a team of diplomats led by Secretary of State John Kerry.
Under that agreement, which the Trump administration abandoned in 2018, Iran shipped most of its nuclear stockpile out of the country, to Russia. The agreement also limited enrichment to under 4 percent.
A year after Mr. Trump pulled out of the nuclear accord, Iran began rebuilding the stockpile and inching up the enrichment levels. The new law is clearly designed as a pressure device aimed at removing economic sanctions, which have crippled Iraq’s economy and choked off much of its oil exports.
Barring international inspectors, the State Department said Wednesday, would violate Iran’s legal obligations under existing nuclear agreements.
“We have full confidence in the I.A.E.A. to continue carrying out its verification and monitoring activities in Iran, and to report to the board of governors if there is any shortfall in Iran’s cooperation,” the State Department said in an emailed statement.
It was not immediately clear how long it would take Iran to reach 20 percent enrichment, but David Albright, president of the Institute for Science and International Security, said it could be done in as little as six months.
“They know how to do it,” Mr. Albright said in an interview. “They’ve done it before. Something less than six months is the worst-case scenario if they devote all their resources to it.”
Mr. Rouhani’s top adviser, Hesameddin Ashena, said that the brief window for Mr. Biden to act after taking office could allow the hard-line faction to take credit for lifting sanctions and for getting Washington to re-enter the nuclear deal by threat instead of by diplomacy.
But it is far from clear that Iran will be willing to simply go back to the old levels. Some officials have demanded that the United States first pay reparations to Iran for lost oil sales since Mr. Trump reimposed sanctions. Some of Mr. Biden’s aides have said that they could not imagine how that could be politically possible.
And while Mr. Biden has said he wants to go back to the deal, he has also said that returning now to the original provisions, which expired in 2030, would be insufficient. The accord would have to be extended and expanded.
The Biden transition team declined to comment on the Iranian law.
“I think the law is a clear sign that Tehran will not be taking a ‘wait and see’ attitude toward Biden’s Iran policy,” said Henry Rome, a senior Iran analyst at Eurasia Group. “Tehran wants to be at the top of the agenda for the new administration, and escalating its nuclear program is a surefire way to do it.”
The concept of expelling inspectors is not a new one. North Korea did the same during a standoff with the Clinton administration in 1994, and also renounced the Nuclear Nonproliferation Treaty, of which it was a member.
In 2006 the North exploded its first nuclear device, and it has conducted a half-dozen or so tests since. Iran and North Korea have cooperated extensively on missile technology in the years since, and Iranian officials have carefully studied the North Korean experience.
Katie Glueck, William J. Broad and Lara Jakes contributed reporting.
