IRAN has begun construction at a secret underground nuclear facility, new satellite photographs reveal.
The ramping up of its capability comes in the wake of it vowing to revenge the killing of nuclear scientist Mohsen Fakhrizadeh, who died in a bomb and gun attack near the capital Tehran.
Iran has promised to "strike like thunder" on whoever carried out the attack – widely believed to be work of an elite Israeli hit squad.
The Islamic Republic has never acknowledged any new building work at its Fordo site, which is built deep inside a mountain to protect it from air strikes.
The new pictures of Fordo emerged via a Twitter account called Observer IL, which cited them as it as coming from South Korea’s Korea Aerospace Research Institute.
The Associated Press said it later contacted the Twitter user, who identified himself as a retired Israeli Defence Forces soldier with a civil engineering background.
He asked that his name not be published over previous threats he received online.
The Korea Aerospace Research Institute acknowledged taking the satellite photo.
“Any changes at this site will be carefully watched as a sign of where Iran’s nuclear program is headed,” said Jeffrey Lewis from the Middlebury Institute of International Studies, who studies Iran.
Shielded by the mountains, the Fordo also is ringed by anti-aircraft guns and other fortifications.
It is about the size of a football field and large enough to house 3,000 centrifuges.
But it is small and hardened enough to lead US officials to suspect it had a military purpose when they exposed the site publicly in 2009.
Under the 2015 nuclear deal, Iran agreed to stop enriching uranium at Fordo and instead make it “a nuclear, physics and technology centre”.
“This location was a major sticking point in negotiations leading to the Iran nuclear deal,” Lewis said.
“The US insisted Iran close it while Iran’s supreme leader said keeping it was a red line.”
Deal or no deal - What was the 2015 Iran nuclear agreement and what has happened to it?
BROKERED by the Obama White House and signed by seven world powers, the Iran nuclear deal aimed to reduce the country's ability to produce nuclear weapons.
However, Donald Trump withdrew the US from the deal - branding it "horrible" and "one-sided".
Iran has also pledged to breach the agreement until it receives the sanctions relief it says it is owed.
The deal was an agreement between the Islamic Republic and a group of world powers aimed at scrapping the Middle Eastern country's nuclear weapons programme.
It saw Iran agree to eliminate its stockpile of medium-enriched uranium by 98 per cent.
Enriched uranium is a critical component for making nuclear weapons and in nuclear power stations and by curbing the amount Iran produce is a way to curb the number of weapons produced.
As part of the agreement, Iran also agreed to only enrich their uranium up to 3.67 per cent over the next 15 years and they agreed to reduce their gas centrifuges for 13 years.
Gas centrifuges are used to separate different types of uranium which allows specific types to then be used to manufacture nuclear weapons or generators.
Iranian nuclear facilities were limited to a single facility with only first-generation centrifuges for 10 years and other nuclear facilities had to be converted into other use.
In addition, they were barred from building any more heavy-water faculties - a type of nuclear reactor which uses heavy water (deuterium oxide) as a coolant to maintain temperatures in the reactor.
Also under the agreement, the International Atomic Energy Agency was granted regular access to all Iranian nuclear facilities to ensure Iran maintains the deal.
If Iran abided by the deal it was promised relief from the US, European Union, and the United Nations Security Council on all nuclear-related economic sanctions.
The agreement was reached on July 14, 2015, and the world powers signed it in Vienna.
But after Donald Trump tore up the 2015 nuclear deal, Iran resumed uranium enrichment – a key process in making nuclear weapons – at the site.
As Iran began ramping up uranium enrichment in in November, tensions began rising, including reported threats by Trump to attack another site.
That culminated in the killing of 59-year-old Fakhrizadeh, as he was being was driven to his mountain retreat in a convoy of three bullet-proof cars.
More than 62 people including snipers and motorbike gunmen were reportedly involved in the precisely-coordinated assassination.
After the scientist’s death a new law approved by its parliament gave paved the way for more uranium enrichment and the blocking UN inspections of nuclear sited.
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This could give Iran the ability to convert its entire stockpile to bomb-grade levels within six months, according to the .
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Earlier this year Iran said it was "ready for war" with the US as it unveiled a new ballistic missile.
It also revealed underground "missile cities" packed full of rockets and explosives.