By Andrew MacAskill, Federico Maccioni and Pesha Magid

LONDON/DUBAI (Reuters) -President Donald Trump said Iran's main nuclear sites had been "obliterated" in military strikes overnight, including on the deeply buried Fordow facility, as the U.S. joined attacks launched by Israel on June 13.

Experts have said military strikes on Iran's uranium enrichment facilities pose limited risks of contamination, and the International Atomic Energy Agency (IAEA) said on Sunday no increased off-site radiation levels had been reported following the U.S. attacks.

WHICH IRANIAN NUCLEAR SITES HAVE BEEN HIT SO FAR?

The U.S. military struck sites at Fordow, Natanz and Isfahan. Trump said Iran's key nuclear enrichment facilities had been "completely and totally obliterated". The attacks follow previously announced Israeli attacks on nuclear sites in Natanz, Isfahan, Arak and Tehran itself.

Israel says it aims to stop Iran building a nuclear bomb and the U.S. says Tehran would not be allowed to get such weapons. Iran denies ever seeking nuclear arms. 

The international nuclear watchdog IAEA has previously reported damage to the uranium enrichment plant at Natanz, the nuclear complex at Isfahan that includes the Uranium Conversion Facility and to centrifuge production facilities in Karaj and Tehran.

Israel has also attacked Arak, also known as Khondab. The IAEA said Israeli military strikes hit the Khondab Heavy Water Research Reactor, which was under construction and had not begun operating, and damaged the nearby plant that makes heavy water.

The IAEA said it was not operational and contained no nuclear material, so there were no radiological effects. Heavy-water reactors can be used to produce plutonium which, like enriched uranium, can be used to make an atom bomb.       

WHAT RISKS DO THESE STRIKES POSE?    

Speaking to Reuters before the U.S. strikes took place, experts said Israel's attacks had posed limited contamination risks so far.

Darya Dolzikova, a senior research fellow at London think-tank RUSI, said attacks on facilities at the front end of the nuclear fuel cycle - the stages where uranium is prepared for use in a reactor - pose primarily chemical, not radiological risks.

At enrichment facilities, UF6, or uranium hexafluoride, is the concern.

"When UF6 interacts with water vapour in the air, it produces harmful chemicals," she said. "In low winds, much of the material can be expected to settle in the vicinity of the facility; in high winds, the material will travel farther, but is also likely to disperse more widely. The risk of harmful chemicals being dispersed is lower for underground facilities."

Simon Bennett, who leads the civil safety and security unit at the University of Leicester in Britain, said risks to the environment were minimal when subterranean facilities are hit because you are "burying nuclear material in possibly thousands of tonnes of concrete, earth and rock".

James Acton, co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, said that before uranium goes into a nuclear reactor it is barely radioactive. "The chemical form uranium hexafluoride is toxic ... but it actually doesn't tend to travel large distances and it's barely radioactive," he added.

Attacks on enrichment facilities were "unlikely to cause significant off-site consequences", he said, while stating his opposition to Israel's campaign.

WHAT ABOUT NUCLEAR REACTORS?

The major concern would be a strike on Iran's nuclear reactor at Bushehr on the Gulf coast.

Fears of catastrophe rippled through the Gulf on June 19 when the Israeli military said it had struck a site in Bushehr, only to say later that the announcement was a mistake.

Israel says it wants to avoid any nuclear disaster.

Richard Wakeford, honorary professor of epidemiology at the University of Manchester, said that while contamination from attacks on enrichment facilities would be "mainly a chemical problem" for the surrounding areas, extensive damage to large power reactors "is a different story".

Radioactive elements would be released either through a plume of volatile materials or into the sea, he added.

Acton of the Carnegie Endowment for International Peace said an attack on Bushehr "could cause an absolute radiological catastrophe".

WHY ARE GULF STATES ESPECIALLY WORRIED?

For Gulf states, the impact of any strike on Bushehr would be worsened by the potential contamination of Gulf waters, jeopardising a critical source of desalinated potable water.

The Gulf Cooperation Council (GCC) is on high alert to monitor for any possible environmental contamination after the attacks, said a source with knowledge of the matter. There have been no signs of radiological contamination so far, the source said, adding that the GCC had emergency plans in place in case of a threat to water and food security in the Gulf.

In the United Arab Emirates, desalinated water accounts for more than 80% of drinking water, while Bahrain became fully reliant on desalinated water in 2016, with 100% of groundwater reserved for contingency plans, authorities say.

Qatar is also 100% dependent on desalinated water.

In Saudi Arabia, a much larger nation with a greater reserve of natural groundwater, about 50% of the water supply came from desalinated water as of 2023, according to the General Authority for Statistics.

While some Gulf states such as Saudi Arabia, Oman and the UAE have access to more than one sea to draw water from, Qatar, Bahrain and Kuwait are crowded along the shoreline of the Gulf with no other coastline.

"If a natural disaster, oil spill, or even a targeted attack were to disrupt a desalination plant, hundreds of thousands could lose access to freshwater almost instantly," said Nidal Hilal, professor of engineering and director of New York University Abu Dhabi's Water Research Center.

"Coastal desalination plants are especially vulnerable to regional hazards like oil spills and potential nuclear contamination," he said.    

(Additional reporting by Tom Perry in Beirut, Crispian Balmer in Jerusalem, Andrew Mills in Doha, Jana Choukeir in Dubai;Writing by Tom Perry; Editing by Andrew Heavens, William Maclean and Helen Popper)