Iran is preparing to retaliate after Israel launched what it called a "preemptive” attack on Iran on Saturday, with the response set to be crushing, an Iranian official told Reuters.

"We had warned you. Now you have started down a path whose end is no longer in your hands," Ebrahim Azizi, the head of the security commission, said separately.

The United States and Israel launched attacks on Saturday, with Israel calling it a "pre-emptive strike" against its arch-enemy "to eliminate threats."

"Our objective is to defend the American people by eliminating imminent threats from the Iranian regime," U.S. President Donald Trump said.

It comes after the U.S. massively increased its military build-up in the region in order to pressure Tehran to scale down its nuclear program, and following a brutal crackdown in Iran on widespread protests.

U.S. President Donald Trump had repeatedly threatened military action if a negotiated solution to the nuclear issue isn't found.

According to eyewitness reports from the Iranian capital Tehran, the residence of religious leader Ayatollah Ali Khamenei was among those hit.

However, he himself is believed to have fled to safety.

Iran's President Masoud Pezeshkian had previously threatened an all-out war if Khamenei was attacked.