Israel struck dozens of Iranian targets early this morning, including nuclear sites, killing top military officials and scientists. The assault raises fears the conflict could explode into all-out war between the Middle East’s two most powerful militaries. Iran had not been attacked by a foreign enemy with such sweeping force since the country was at war with Iraq in 1989.

The strikes appear to have all but wiped out Iran’s chain of command, with Iranian officials and media reports saying that at least three top generals were killed, as well as two prominent nuclear scientists, in what appeared to be a move away from individual assassinations to targeted strikes. At least a dozen civilians were also killed, according to the state news agency.

Several Israeli officials described the attacks as a pre-emptive strike intended to keep Iran from acquiring a nuclear bomb. The Israeli military suggested there would be more to come, calling the strike “the first stage.” Israel’s prime minister, Benjamin Netanyahu, vowed that the battle would continue for “as many days as it takes.”

The assault came one day after the UN’s nuclear watchdog censured Iran for violating nonproliferation obligations and as American and Iranian negotiators planned Sunday talks in Oman about Iran’s nuclear program — meetings that are now in doubt. The U.S. said it was not involved in the strikes. It may take days, or weeks, to assess how far Israel has set back Iran’s atomic capabilities.

Next steps: Iran’s supreme leader, Ayatollah Ali Khamenei, said in a statement that Israel “should anticipate a harsh punishment. The strong hand of the Islamic Republic will not let them go.” Iranian fighter jets moved quickly to intercept the Israeli warplanes, an Iranian official said, and sirens wailed in Jerusalem and other cities.

Witness accounts: Residents of Tehran, the Iranian capital, reported hearing huge explosions, and Iranian state television broadcast images of smoke and fire billowing from buildings.