Iran must “immediately stop and reverse” its plans to raise its enrichment of uranium, breaking another limit of its 2015 nuclear deal with world powers, the UK Government has demanded.
Iranian Government spokesperson Ali Rabiei said on Sunday that his country will go beyond the limit of 3.67% enrichment and that the new percentage “will be based on our needs”.
Iran made the decision a year after President Donald Trump unilaterally withdrew the US from landmark Iran nuclear deal.
Iran has repeatedly warned Europe in recent weeks that it would begin walking away from an accord neutered by an American campaign of sanctions that blocked Tehran’s oil sales abroad and targeted its top officials.
The decision came less than a week after Iran acknowledged breaking the deal’s 300 kilogram limit on its low-enriched uranium stockpile.
Where do the escalating tensions stem from?
In 2015, Iran agreed a long-term deal on its nuclear programme with the US, UK, France, China, Russia and Germany.
Under the accord, Iran agreed to limit its sensitive nuclear activities and allow in international inspectors in return for the lifting of crippling economic sanctions.
It came after years of tension over Iran's alleged efforts to develop a nuclear weapon.
Iran insisted that its nuclear programme was entirely peaceful, but the international community did not believe that.
However, in 2018 Mr Trump pulled the US out of the deal which he called "terrible" and imposed more sanctions on Iran, crippling the country's economy.
What role does uranium play?
The nuclear deal was intended to stop Iran enriching uranium, which although it can be used for nuclear power, can also be used to develop nuclear weapons.
After the US pulled out of the nuclear deal and imposed sanctions on Iran, the country did not see the benefits of sticking to it, and hoped to push European powers towards helping the easing of the strict conditions.
What is uranium enrichment?
Enriching a supply of uranium means altering its chemical structure to boost its concentration.
Depending on how much uranium is enriched, it can be used to fuel nuclear reactors, or can be turned into chemical weapons.
Iran maintains its nuclear programme is for peaceful purposes.
What did Iran announce on Sunday?
At a news conference, Iranian officials said the new level of uranium enrichment would be reached later in the day, but did not provide the percentage they planned to hit.
Under the nuclear deal, the cap for enrichment was set at 3.67%.
"Within hours, the technical tasks will be done and enrichment above 3.67% will begin," Iran nuclear agency spokesperson Behrouz Kamalvandi said.
He added the Middle Eastern nation expected to pass this point by Monday morning.
Ali Akbar Velayati, an aide to Supreme Leader Ayatollah Ali Khamenei, said on Saturday that Iran needed uranium enriched to 5%.
Bushehr, Iran's only nuclear power plant, is now running on imported fuel from Russia that's enriched to around 5%.
Iranian Foreign Minister Mohammad Javad Zarif sent a letter to EU foreign policy chief Federica Mogherini outlining the steps it had taken, said Abbas Araghchi, a deputy foreign minister.
Discussions with European powers are continuing and ministerial-level talks are planned later this month, he said.
"We will give another 60-day period, and then we will resume the reduction of our commitments," Mr Araghchi said, without elaborating.
What has the response been from the UK?
In response to Iran's announcement, the UK Foreign Office (FCO) said Iran must "immediately stop and reverse" all activities which have put it in breach of its nuclear deal.
"While the UK remains fully committed to the deal, Iran must immediately stop and reverse all activities inconsistent with its obligations," a spokesperson said.
"We are co-ordinating with other JCPOA participants regarding the next steps under the terms of the deal, including a joint commission."
What have others said?
Experts warn higher enrichment and a growing stockpile narrow the one-year window Iran would need to have enough material for an atomic bomb, something Iran denies it wants but the deal prevented.
In a last-minute diplomatic bid, French President Emmanuel Macron spoke to his Iranian counterpart, Hassan Rouhani, by phone, saying he is trying to find a way by July 15 to resume dialogue between Iran and Western partners.
Can the nuclear deal be saved?
Hopes for saving the faltering deal appear increasingly dim, as the Europeans have been unable to offer Iran any effective way around US sanctions.
While the steps are concerning to nuclear non-proliferation experts, they could be easily reversible if Europeans offer Iran the sanctions relief it seeks.
What else has happened?
Tensions began rising in May when the US rushed thousands of additional troops, an aircraft carrier, nuclear-capable B-52 bombers and advanced fighter jets to the Middle East.
Mysterious oil tanker blasts near the Strait of Hormuz, attacks by Iranian-backed rebels in Yemen on Saudi Arabia and Iran shooting down a US military drone have raised fears of a wider conflict engulfing a region crucial to global energy supplies.
What impact has increased tensions had on Iran?
It's not good for Iran.
Following the US's withdrawal from the nuclear agreement in 2015, America began ramping up sanctions on Tehran.
Other countries which deal with Iran are also being penalised, leading to other nations turning their back on Tehran.
Iran's economy, which relies heavily on oil, has been crippled as a result.
Supporters of the regime say Iran's increased nuclear activity is a reaction to the outside influences put on its economy by the US.
Iranian President Hassan Rouhani said America's "hostility directly targets every single Iranian", and admitted the "low income and weak classes of the society are undergoing more pressure compared to other classes".
He added: "Iran has been loyal to its commitments, to its signature and international agreements.
"The one who is standing up to us today [referring to US president Donald Trump] has stomped on promises, agreements and international regulations."
A report by the World Bank found Iran's economy continued to slow down in the first quarter of 2018/19 as the US sanctions were reimposed.