Israel has launched a significant assault on Iran, bringing their long-standing covert conflict into a more open and potentially expansive regional war.

Early Friday, explosions rocked Tehran’s capital as Israel claimed to target Iranian military and nuclear sites. Iranian state media announced that the head of the Revolutionary Guard and two senior nuclear scientists were killed.

Israel’s attack comes as tensions have escalated over Iran’s rapidly advancing nuclear program, which Israel sees as a threat to its existence.

The Trump administration renewed attempts to negotiate limits on Iran’s uranium enrichment in return for easing economic sanctions. However, indirect discussions between American and Iranian diplomats have reached an impasse.

The attack pushed the region into a new and uncertain phase. Here’s what to know about the strikes:

Israel hit nuclear sites, killed Revolutionary Guard chief

Israeli leaders said the attack was aimed at preventing Iran from acquiring a nuclear bomb as the country enriches uranium a short, technical step away from weapons-grade levels. Iran long has said its program is peaceful and U.S. intelligence agencies have assessed Iran was not actively building a weapon.

In a video announcing the military operation, Israeli Prime Minister Benjamin Netanyahu said the strikes hit Iran’s main enrichment site, the Natanz atomic facility, and targeted Iran’s leading nuclear scientists. He said that Israel had also targeted Iran’s ballistic missile arsenal.

Iranian state TV reported that the head of Iran’s Revolutionary Guard and one of Iran’s most important commanders, Gen. Hossein Salami, had been killed.

Residents of Tehran reported hearing huge explosions. Iranian state TV broadcast footage of blown-out walls, burning roofs and shattered windows in residential buildings across the capital. It reported that blasts had set the Revolutionary Guard’s headquarters ablaze.

Bracing for retaliation, Israel closed its airspace and said it was calling up tens of thousands of soldiers to protect the country’s borders.

Unclear how close Iran is to building a bomb

Netanyahu claimed Friday that if Iran wasn’t stopped, “it could produce a nuclear weapon within a very short time.” But it likely would take Iran months to build a weapon, should it choose to do so. It also hasn’t proved its ability to miniaturize a bomb to be placed atop missiles.

Iranian officials have openly threatened to pursue the bomb. Tensions over Iran’s rapid nuclear advances and growing reserves of highly enriched uranium are surging seven years after President Donald Trump unilaterally withdrew the U.S. from Tehran’s 2015 nuclear deal with world powers.

For the first time in two decades, the atomic watchdog agency on Thursday censured Iran for failing to comply with nuclear nonproliferation obligations meant to prevent it from developing a nuclear weapon.

In response, Iran said that it would open a previously undisclosed enrichment site and accelerate production of 60% highly enriched uranium, which could be easily processed to the 90% level used in nuclear weapons.

Iran’s nuclear sites have long been a flash point

Iran has two main enrichment sites, Natanz, in central Isfahan province, and Fordo, near the Shiite holy city of Qom, some 90 kilometers (55 miles) southwest of Tehran.

Both are designed to protect from potential airstrikes. Natanz is built underground on Iran’s Central Plateau, and has been targeted several times in suspected Israeli sabotage attacks, as well as by the Stuxnet virus, believed to be an Israeli and American creation, which destroyed Iranian centrifuges.

Fordo is buried deep inside mountain and protected by anti-aircraft batteries. It also hosts centrifuge cascades, but isn’t as big a facility as Natanz.

Both sites have been the focus of the Trump administration’s recent push to negotiate a new nuclear deal with Tehran. Trump said that he warned Netanyahu against launching an attack on Iran’s nuclear facilities while diplomatic efforts were underway.

U.S. special envoy Steve Witkoff was expected to meet his Iranian counterparts in Oman for a sixth round of negotiations to start Sunday. It wasn’t clear if those talks were still on.

Iran threatens retaliation

Hours after the strikes, Iran’s Supreme Leader Ayatollah Ali Khamenei threatened Israel would face “severe punishment.”

“The powerful hand of the armed forces of the Islamic republic will not let (the attacks) go unpunished,” the leader added in a statement posted online.

Other Iranian officials echoed his warning, pledging vengeance. State TV aired footage of Iranians chanting “Death to Israel!” and “Death to America!”

From Washington, Trump said that the U.S. had not been involved in the attack and warned Iran against retaliations against American interests in the region.