Israel said it hit hundreds of targets on Friday alone, and has since continued its attacks, striking a defence facility and fuel depots on Sunday.

The fiercest-ever exchange of fire between the arch foes came in the midst of ongoing talks between Tehran and Washington seeking to reach a deal on Iran's nuclear programme.

"Israel has been following the nuclear programme for 15 years," Israeli analyst Michael Horowitz told AFP.

"The strikes we see are the result of years of intelligence gathering and infiltration of the Islamic republic."

Israel has previously carried out more limited attacks against Iranian military sites, including in October last year.

"It seems that this week's operation had been in preparation for months, with an acceleration after the operation last year significantly weakened Iranian air defences," Horowitz said.

The United States and other Western countries, along with Israel, accuse Iran of seeking a nuclear weapon.

Tehran denies that, but has gradually broken away from its commitments under a 2015 nuclear deal it struck with world powers, which the United States withdrew from during President Donald Trump's first term.

The landmark accord had provided Iran sanctions relief in exchange for curbs on its atomic programme.

- 'Impressive' precision -

Israel is long believed to have carried out sabotage operations inside Iran through its Mossad espionage agency, but the attacks that started on Friday are by far the broadest in scope.

The campaign has shown Israel to be capable of precisely locating and taking out high-ranking Iranian officials and nuclear scientists.

A European security source said the strikes showed "an impressive degree of precision and mastery".

Israel says it has killed 20 military and security targets, including the armed forces' chief of staff and the head of Iran's Revolutionary Guards, as well as nine nuclear scientists.

Danny Citrinowicz, of the Tel Aviv-based Institute for National Security Studies, said the ability to take out high-ranking officials showed "intelligence, but also operational superiority of Israel over Iran".

"It's highly connected to the ability to collect information, intelligence in Iran for years, excellent intelligence that actually allows you to detect where they're living, the seniors, also the nuclear scientists," he told AFP.

The latest targeted strikes come after Israel killed Ismail Haniyeh, the political leader of the Palestinian militant group Hamas, in a Tehran guesthouse in July last year.

Haniyeh had been in town for the inauguration of Iranian President Masoud Pezeshkian, and was reportedly killed by an explosive device that Israeli operatives had placed in the guesthouse weeks before.

Analysts at the time saw the attack as a major intelligence failure by Iran.

But "still Iran didn't learn and didn't have the ability to lock the holes in their system", said Citrinowicz.

- Smuggled drones -

News site Axios reported that Mossad agents inside Iran helped prepare Friday's strikes, pre-positioning weapon systems near air defence targets, including by hiding them inside vehicles, and launching them when the attack started.

An Israeli intelligence official told Axios that Mossad had "established an attack drone base inside Iran with drones that were smuggled in long before the operation".

A former Israeli intelligence official told The Atlantic that "Iranians opposed to the regime make for a ready recruiting pool", and that "Iranians working for Israel were involved in efforts to build a drone base inside the country".

Alain Chouet, the former number three in the French intelligence, said he was convinced Israel had "half a dozen cells capable of acting at any moment" inside Iran.

Iran regularly executes people it accuses of spying for Israel.

It is unclear what role, if any, Israel's key military and diplomatic ally the United States had in the latest strikes.

Before the attack early on Friday, Trump publicly urged Israel to allow time for diplomacy, as a sixth round of US-Iranian nuclear talks were set to be held in Oman at the weekend.

The US president on Sunday said Washington "had nothing to do" with ally Israel's campaign.

But Iran's Foreign Minister Abbas Araghchi on Sunday said Tehran had "solid proof of the support of the American forces and American bases in the region" for the attacks.

And Israel's Prime Minister Benjamin Netanyahu said on Saturday his country was acting with "the clear support of the president of the United States".