The US reportedly wants Iran to stop enriching for 20 years. Iran has agreed to only five years. Here’s why that matters.

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As the diplomatic push to renew ceasefire talks between the United States and Iran continues, Washington and Tehran have also been negotiating a deal on Iran’s uranium enrichment.

Iran’s current stockpile of enriched uranium — and its ability to enrich further — have been central elements of the long-standing demand by the administration of US President Donald Trump that Tehran not only commit to not building a nuclear weapon, but also give up the ability to do so.

Now, according to multiple US media reports, specific differences regarding Iran’s nuclear enrichment have emerged as a critical stumbling block in negotiations between the two countries aimed at ending their war.

High-powered teams from the US and Iran met in Islamabad, Pakistan, over the weekend, but failed to agree to a deal. Pakistan is attempting to get both sides to come to a second round of talks.

The breakdown in talks in Pakistan over the weekend occurred over a US insistence that Iran suspend its uranium enrichment programme for 20 years in exchange for sanctions relief, followed by Tehran’s refusal to agree to a moratorium on enrichment beyond five years.

So why are Iran and the US arguing over the duration of uranium enrichment? Will it impact ceasefire talks?

Here’s what we know:

Uranium is a naturally occurring radioactive material found in rocks, soil, and water, which, when enriched, is used as a source of fuel for nuclear reactors.

It comes in the form of three natural isotopes (chemical elements that contain equal numbers of protons but different numbers of neutrons in their nuclei): uranium-234 (U-234), uranium-235 (U-235) and uranium-238 (U-238). U-235 is highly radioactive; the other isotopes are not.

According to the United Nations’ atomic energy watchdog, the International Atomic Energy Agency (IAEA), uranium enrichment is “the process through which the isotopic proportion of U-235 is increased from 0.72 percent to up to 94 percent”.

The IAEA says uranium is considered low-enriched if its isotopic proportion of U-235 remains below 20 percent. This is generally used in civilian, commercial reactors to generate power for homes and industries. It is considered highly enriched if its isotopic proportion is beyond 20 percent.

Enrichment needs to cross 90 percent for it to be considered weapons-grade.

Uranium also needs to be in the form of a gas to carry out the enrichment process, and most countries, including Iran, use the process of spinning uranium hexafluoride in order to enrich it. This involves feeding the gas into fast-spinning centrifuges. The lighter U-235 separates from the heavier U-238.

The useful enriched uranium-235 is then captured for nuclear fuel.

Currently, Iran is believed to have about 440kg (970 pounds) of uranium enriched to 60 percent – the level at which it becomes much faster to get to the 90 percent threshold needed to produce a nuclear weapon.

That amount is enough, theoretically, to produce more than 10 nuclear warheads, IAEA chief Rafael Grossi told Al Jazeera in early March. He added that almost half of the 60-percent-enriched uranium was probably still in an underground tunnel complex at Iran’s Isfahan nuclear facility.

An unknown quantity is also believed to be stored at the Natanz facility. These two underground nuclear sites, along with a third at Fordow, were destroyed or badly damaged in US-Israeli air strikes in the 12-day war in June 2025, and have been targeted again during the current conflict.

It is unclear whether these stockpiles are buried under rubble — and what condition the enriched uranium is in.

Iran has always maintained that its uranium enrichment programme is for civilian energy purposes only, despite having enriched uranium far beyond the threshold required for that.

Israel and the US have repeatedly accused Iran of enriching uranium to develop nuclear weapons. The US and its allies, especially Europe, have slapped multiple rounds of sanctions on the country.

In 2015, Iran struck a pact with world powers negotiated by then-US President Barack Obama, called the Joint Comprehensive Plan of Action (JCPOA). Under the pact, Tehran agreed to scale down its nuclear programme in exchange for relief from sanctions.

But in 2018, during his first term as president, Trump pulled the US out of the pact, calling it “one-sided” and reimposing sanctions on Iran.

Since then, Trump has repeatedly said that Iran should not have the ability to produce nuclear weapons. It has been one of Washington’s main stated demands during talks with Iranian officials over the past year, and was also the central justification that Washington used when it bombed Iranian nuclear facilities during the 12-day war last year.

It is also one of the reasons the US stated while starting the war on Iran in February of this year, despite being in active negotiations over uranium enrichment with the country at the time.

Trump has reportedly been considering dispatching US special forces to Iran to seize the country’s stockpile of highly enriched uranium.

Iranian officials have stated they are open to discussing reducing the level of enrichment in past negotiations, but have refused to dismantle Iran’s nuclear programme entirely.

According to US media outlets, the ceasefire talks in Pakistan’s capital Islamabad collapsed without a deal due to differences over Iran’s nuclear programme, with Washington pushing for a 20-year suspension and Iran proposing a five-year one.

MV Ramana, professor in Disarmament, Global and Human Security at the University of British Columbia, told Al Jazeera that the current discussion about how long Iran should suspend enriching uranium is primarily a process of bargaining.

“Both the US and Iranian governments have staked a lot of political capital on Iran’s enrichment programme. The dispute in recent years and months has been that Trump wanted Iran to completely abandon the programme, and Iran refusing to ever do that,” he said.

“Now they have moved a little from that position and are trying to stake out positions that are somewhat closer to each other. Beyond that, I cannot discern any technical reasons for either 20 or five years,” he added.

Under the JCPOA, Iran agreed to cap its U-235 enrichment levels to 3.67 percent — levels enough to fuel civil nuclear plants like the one at Bushehr, but far below what is needed for weapons — for 15 years. It also agreed not to build new centrifuges for 10 years, and to reduce the number of existing centrifuges over that decade. Enrichment — up to 3.67 percent — was only permitted at the Natanz facility. And Iran’s enriched uranium stockpile would be kept under 300kg (660 pounds).

At the point when Trump unilaterally withdrew from the JCPOA, Iran was fully compliant with its commitments, according to the IAEA and US intelligence agencies.

Eight years later, the JCPOA’s terms might be at least partly responsible for the wrangling over the duration for which Iran agrees to cap its enrichment programme, said Ian Lesser, distinguished fellow at the German Marshall Fund of the United States.

“At the time, he [Trump] and other critics felt the 10-year horizon was too short. So it is not surprising that both sides should pursue presenting a ‘better deal’ from their perspectives,” Lesser said.

In other words, Iran wants to commit to a period of less than 10 years, and the US to a period longer than a decade.

After the first round of ceasefire talks concluded without a deal over the weekend, US Vice President JD Vance — who led his country’s delegation — told journalists that “the ball is in the Iranian court”.

“It’s one thing for the Iranians to say that they’re not going to have a nuclear weapon. It’s another thing for us to put in place the mechanism to ensure that’s not going to happen,” he said.

Chris Featherstone, a political scientist at the University of York, told Al Jazeera that Iran is understandably interested in offering the fewest concessions to the US in their negotiations.

“For the US, they have long held the view that Iran should be prevented from obtaining nuclear weapons. As such, they want to ensure Iran commits to not enriching uranium for as long as possible. Importantly, the longer that Iran goes without enriching uranium, the more difficult it is to restart the process,” Featherstone explained.

He added that for Trump, it’s also all about being able to justify his claims that he is winning the war.

“Iran committing to not enriching uranium for as long as possible can be seen as a ‘success’,” Featherstone said. “And he can demonstrate that he has achieved something with this war.”