Israel's attacks across Iran on Friday, which targeted "dozens" of sites including nuclear facilities, military commanders and scientists, are manifestly illegal, leading scholars of international law have said.

Accusing the government in Tehran of beginning to build nuclear warheads, Israeli Prime Minister Benjamin Netanyahu said the attack was aimed at "rolling back the Iranian threat to Israel's very survival", adding that it would take "many days".

"We struck at the heart of Iran's nuclear enrichment programme," Netanyahu said in a recorded televised address.

"We targeted Iran's main enrichment facility in Natanz. We targeted Iran's leading nuclear scientists working on the Iranian bomb. We also struck at the heart of Iran's ballistic missile programme."

Netanyahu's decision is premised on "preventive self-defence" arguments, which justify the use of force against another state to prevent an anticipated future attack.