Iran has started construction on a second nuclear reactor at a power plant that Tehran cites as its reason for breaking the enrichment limit set by the unravelling 2015 nuclear deal with world powers.

Officials celebrated the start of construction at the Bushehr facility, but the politics of the moment were not lost on them as a US pressure campaign of sanctions blocks Tehran from selling crude oil abroad.

“It was not us who started breaking commitments, it was them who did not keep to their commitments and cannot accept the nuclear deal atrs a one-way road map,” said Ali Akbar Salehi, head of the Atomic Energy Organisation of Iran.

Bushehr is fuelled by uranium produced in Russia and is monitored by the United Nations’ International Atomic Energy Agency (IAEA).

In photos: British-flagged tanker seized by Iran Show all 13 1 /13 In photos: British-flagged tanker seized by Iran In photos: British-flagged tanker seized by Iran A speedboat of Iran's Revolutionary Guard moving around the British-flagged oil tanker Stena Impero Tasnim News Agency/AP In photos: British-flagged tanker seized by Iran Crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran A satellite image of the Iranian port city of Bandar Abbas reveals the presence of the seized British oil tanker, the Stena Impero on July 22 Maxar Technologies/Reuters In photos: British-flagged tanker seized by Iran Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran Iranian Revolutionary Guards patrolling around the captured British-flagged tanker Stena Impero as it's anchored off the Iranian port city of Bandar Abbas AFP/Getty In photos: British-flagged tanker seized by Iran epa07733612 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran A satellite image of the Iranian port city of Bandar Abbas reveals the presence of the seized British oil tanker, the Stena Impero on July 22 Maxar Technologies/Reuters In photos: British-flagged tanker seized by Iran A speedboat of the Iran's Revolutionary Guard moves around a British-flagged oil tanker Stena Impero AP In photos: British-flagged tanker seized by Iran epa07733614 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran In this photo released by state-run IRIB News Agency, which aired on Monday, July 22, 2019, shows cooks in the galley of the British-flagged tanker Stena Impero that was seized by Tehran in the Strait of Hormuz on Friday. The Associated Press cannot independently verify the condition of the crew members, but in the video they looked to be in good health and it didn't appear as though they were being filmed under duress. (IRIB News Agency via AP) AP In photos: British-flagged tanker seized by Iran epa07733619 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran epa07733613 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency In photos: British-flagged tanker seized by Iran epa07733616 A handout photo made available by Iranian state-run IRIB News Agency on 22 July 2019 shows crew members of the British-flagged tanker Stena Impero, that was seized by Iran in the Strait of Hormuz on 19 July 2019. Iranian Revolutionary Guard Corps (IRGC) claims to have seized Stena Impero at the Strait of Hormuz with 23 crew on board on 20 July 2019. EPA/IRIB NEWS AGENCY / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES IRIB NEWS AGENCY / HANDOUT EPA/IRIB News Agency

But Iran began 4.5 per cent enrichment in part to supply Bushehr, despite the deal limiting it to 3.67 per cent.

While this is still nowhere near weapons-grade levels of 90 per cent, nonproliferation experts warn that Iran’s growing stockpile and increasing enrichment will begin to shave off time from the estimated year Tehran would need to gather enough material for an atomic bomb.

Iran has long maintained its enrichment is for peaceful purposes, though the deal was designed to limit its program in exchange for the lifting of international sanctions.

Those limits blocked its path to being able to have enough material for a bomb.

Concrete was poured into the prepared base of the second reactor in Bushehr, which is 440 miles south of Tehran, as journalists watched on Sunday.

Officials say the new reactor, and a third planned to be built, will each add more than 1,000 megawatts to Iran’s power grid.

It is being built with assistance from Russia, which helped bring Bushehr’s first reactor online in 2011 after decades of delays.

Bushehr (above) is fuelled by uranium produced in Russia and is monitored by the United Nations’ International Atomic Energy Agency (Getty)

Mr Salehi praised the plant’s operations, saying: “The security of this power plant has been provided by the armed forces, and its safety has been endorsed by international institutions.”

This comment appeared to be a dig at Gulf Arab states opposed to Tehran, who earlier raised concerns to the IAEA that Bushehr was a risk to the wider region over earthquakes that routinely hit Iran.

Meanwhile, Iran’s foreign ministry spokesman rejected claims by the US and Israel over allegations of nuclear material being discovered at an undeclared site outside Tehran.

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An IAEA meeting last week appeared to include discussions over what Benjamin Netanyahu, the Israeli prime minister, described in a United Nations speech last year as a “secret atomic warehouse”.

The IAEA has said Iran “carried out activities relevant to the development of a nuclear explosive device” in a “structured program” through to the end of 2003.

Israeli officials allege material recovered from the warehouse came from that program.

“The Zionist regime and others are trying to re-open this case. We don’t accept this and we condemn these efforts,” Iran’s foreign ministry spokesman said.