By JON GAMBRELL, Associated Press

DUBAI, United Arab Emirates (AP) — U.S. forces have attacked three Iranian nuclear and military sites, further upping the stakes in the Israel-Iran war.

President Donald Trump said the strikes, which he described as “very successful,” had hit the Natanz, Fordo and Isfahan sites, with Fordo being the primary target. Iran’s state-run IRNA news agency confirmed there were attacks early Sunday at all three nuclear sites.

Israel launched a surprise barrage of attacks on sites in Iran on June 13, which Israeli officials said was necessary to head off what they claimed was an imminent threat that Iran would build nuclear bombs.

Iran, which has long insisted its nuclear program is peaceful, has retaliated with a series of missile and drone strikes in Israel, while Israel has continued to strike sites in Iran.

The U.S. and Iran had been in talks that could have resulted in the U.S. lifting some of its crushing economic sanctions on Iran in exchange for Tehran drastically limiting or ending its enrichment of uranium. Until Saturday, Washington had helped shoot down Iranian strikes on Israel but had not launched direct attacks on Iran.

Here’s a look at the sites Trump said the U.S. struck and their importance to Iran’s nuclear program.

Natanz enrichment facility

Iran’s nuclear facility at Natanz, located some 135 miles southeast of Tehran, is the country’s main enrichment site and had already been targeted by Israeli airstrikes. Uranium had been enriched to up to 60% purity at the site — a mildly radioactive level but a short step away from weapons grade — before Israel destroyed the aboveground part of the facility, according to the U.N.’s nuclear watchdog, the International Atomic Energy Agency.

Another part of the facility on Iran’s Central Plateau is underground to defend against potential airstrikes. It operates multiple cascades, or groups of centrifuges working together to more quickly enrich uranium. The IAEA has said it believes that most if not all of these centrifuges were destroyed by an Israeli strike that cut off power to the site.

The IAEA said those strikes caused contamination only at the site itself, not the surrounding area.

Iran also is burrowing into the Kūh-e Kolang Gaz Lā, or Pickax Mountain, which is just beyond Natanz’s southern fencing. Natanz has been targeted by the Stuxnet virus, believed to be an Israeli and American creation, which destroyed Iranian centrifuges. Two separate attacks, attributed to Israel, also have struck the facility.

Fordo enrichment facility

Iran’s nuclear facility at Fordo is located some 60 miles southwest of Tehran. It also hosts centrifuge cascades, but isn’t as big as Natanz. Its construction began at least in 2007, according to the IAEA, although Iran only informed the U.N. nuclear watchdog about the facility in 2009 after the U.S. and allied Western intelligence agencies became aware of its existence.

Buried under a mountain and protected by anti-aircraft batteries, Fordo appears designed to withstand airstrikes. Military experts have said it could likely only be targeted by “bunker buster” bombs — a term for bombs that are designed to penetrate deep below the surface before exploding — such as the latest GBU-57 A/B Massive Ordnance Penetrator bomb in the American arsenal. The roughly 30,000 pound precision-guided bomb is designed to attack deeply buried and hardened bunkers and tunnels.

The U.S. has only configured and programed its B-2 Spirit stealth bomber to deliver that bomb, according to the Air Force. The B-2 is only flown by the Air Force, and is produced by Northrop Grumman, meaning that Washington would have to be involved in such an operation.

Isfahan Nuclear Technology Center

The facility in Isfahan, some 215 miles southeast of Tehran, employs thousands of nuclear scientists. It also is home to three Chinese research reactors and laboratories associated with the country’s atomic program.

Israel has struck buildings at the Isfahan nuclear site, among them a uranium conversion facility. The IAEA said there has been no sign of increased radiation at the site.

Other nuclear sites

Iran has several other sites in its nuclear program that were not announced as targets in the U.S. strikes.

Iran’s only commercial nuclear power plant is in Bushehr on the Persian Gulf, some 465 miles south of Tehran. Iran is building two other reactors like it at the site. Bushehr is fueled by uranium produced in Russia, not Iran, and is monitored by the IAEA.

The Arak heavy water reactor is 155 miles southwest of Tehran. Heavy water helps cool nuclear reactors, but it produces plutonium as a byproduct that can potentially be used in nuclear weapons. Iran had agreed under its 2015 nuclear deal with world powers to redesign the facility to relieve proliferation concerns.

The Tehran Research Reactor is at the headquarters of the Atomic Energy Organization of Iran, the civilian body overseeing the country’s atomic program. It initially required highly enriched uranium but was later retrofitted to use low-enriched uranium over proliferation concerns.

Associated Press staff writer Abby Sewell in Beirut contributed to this report.

The Associated Press receives support for nuclear security coverage from the Carnegie Corporation of New York and Outrider Foundation. The AP is solely responsible for all content.