Iran says it has breached further commitments to the 2015 nuclear deal by enriching uranium to a purity level beyond what has been internationally agreed.
Exceeding this limit not only marks a violation in the treaty, but it also enforces Iran's adamant reaction to the US reintroducing crippling sanctions after backing out of the deal last year.
The first reaction came last week as Foreign Minister Mohammad Javad Zarif told local media that Iran's stockpile of enriched uranium had exceeded 300kg, which was the limit originally agreed in the deal.
But what is the significance of this percentage, and how far away is this from a nuclear weapon?
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How is uranium enriched?
Only a very specific isotope of uranium — uranium 235 — can be used for nuclear energy or weapons, and to reach it requires a process of extraction.
The uranium 235 is isolated by using centrifuges to take advantage of the differing weights of other uranium isotopes.
For example, uranium 235 is lighter than uranium 238 — an isotope usually in abundance.
After being converted into a gas and placed in a centrifuge, the heavy and undesirable uranium isotopes are pulled to the edge, leaving the lighter isotopes closer to the centre.
A small amount of a more-refined uranium 235 can then be extracted, but the process would need to be repeated over and over to continue to increase the concentration.
What is the significance of the various percentages of purity?
Nuclear reactors generally use enriched uranium with a purity level between 3-5%.
Under the 2015 nuclear deal, Iran agreed to phase out a large number of its centrifuges and limit the purity of its enriched uranium to 3.67%.
But on Monday it said it had begun enriching uranium to 4.5%.
To put this into perspective, a concentration level below 20% is considered a low-enriched uranium (LEU), while anything above this is a highly-enriched uranium (HEU).
To reach a level considered weapons-grade, the purity level would need to be around 80-90%.
It is still possible to create a nuclear weapon with a lower concentration than this, but the weapon would be much larger, heavier and difficult to move.
If Iran has only increased to 4.5%, and a weapons-grade purity is 80%, why are people still talking about nuclear weapons?
Despite there being a huge gap between 4.5% and 80% in figures, it is not actually that large a gap when taking into account the intricacies of the enrichment process.
The most laborious part of enrichment is between 0-20% — once this level is reached, enrichment can boost to 90% quite rapidly.
According to The Associated Press, there is one uranium 235 atom for every 140 atoms of uranium 238 in a freshly-mined product.
In order to reach Iran's limit of 3.67%, 114 of the undesirable uranium 238 isotopes must be extracted.
But after reaching a 20% purity level, there is just four uranium 238 atoms to every single uranium 235.
This means that these remaining four undesirables must be extracted to reach 90% concentration.