Fears of an all-out war between Israel and Iran are increasing, after Tehran hit back at deadly strikes on Friday with a mass drone attack.

It had already threatened “harsh and decisive” retaliation after Israel struck Tehran early on Friday (local time) in attacks that appeared to be the most significant attack Iran has faced since its 1980s war with Iraq.

Multiple sites were hit across the country.

Later on Friday, the Israel Defence Force said Tehran had launched more than 100 unmanned drones toward Israel.

“All [aerial] defence arrays have been operating to intercept the threats,” military spokesperson Brigadier General Effie Defrin said.

“This is a different event to what we’ve experienced thus far, and we’re expecting difficult hours.”

Israel’s airspace remained closed after it launched its Iran strikes on Friday morning.

Among those killed in the attacks were the leader of Iran’s paramilitary Revolutionary Guard, General Hossein Salami, Iranian Armed Forces chief of staff Mohammad Bagheri (the country’s Iran’s highest-ranking military officer) and former national security chief Ali Shamkhani (a key adviser to Iran’s Supreme Leader Ayatollah Khamenei. Two nuclear scientists also died.

Iran’s Supreme Leader Ayatollah Ali Khamenei was alive and was being continuously briefed about the situation, a security source told Reuters.

Khamenei said Israel had committed a “crime at dawn” and claimed it had struck residential areas.

“The regime must await a severe punishment,” he said in a statement released by the Iranian state media agency, IRNA.

“With this crime, the Zionist regime has prepared a bitter and painful fate for itself – and it will certainly receive it.

“In the enemy’s attacks, a number of commanders and scientists were martyred. Their successors and colleagues will, God willing, immediately continue their duties without pause.”

Israeli leaders said the attack was necessary to head off what they described as an imminent threat that Iran would build nuclear bombs, and they warned of a reprisal that could target civilians in Israel.

Multiple sites in Tehran were hit in the attack, which Israeli Prime Minister Benjamin Netanyahu said targeted nuclear and military sites.

“Moments ago, Israel launched operation Rising Lion, a targeted military operation to roll back the Iranian threat to Israel’s very survival,” he said in a televised address.

“This operation will continue for as many days as it takes to remove this threat.”

Also targeted were officials leading Iran’s nuclear program and its ballistic missile arsenal.

The assault came amid warnings from Israel that it would not permit Tehran to build a nuclear weapon, though it remains unclear how close the country actually is to achieving that.

In Washington, the Trump administration, which earlier cautioned Israel against an attack amid continuing negotiations, said it had not been involved in the attack and warned Iran against retaliations against US interests or personnel.

The attack followed increasing tensions that led the US to pull some diplomats from Iraq’s capital and to offer voluntary evacuations for the families of US troops in the wider Middle East.

US Secretary of State Marco Rubio said Israel took “unilateral action against Iran” and Israel advised the US that it believed the strikes were necessary for its self-defence.

“We are not involved in strikes against Iran, and our top priority is protecting American forces in the region,” Rubio said in a statement that warned Iran against targeting US interests or personnel.

The attack comes as tensions have reached new heights over Tehran’s rapidly advancing nuclear program.

Benchmark Brent crude spiked on news of the attack, rising nearly 5 per cent.

Both Iran and Israel closed their airspace.

As the explosions in Tehran started, US President Donald Trump was on the lawn of the White House mingling with members of Congress.

It was unclear if he had been informed.

Trump earlier said he had urged Netanyahu to delay taking action while his administration negotiated with Iran.

“As long as I think there is a [chance for an] agreement, I don’t want them going in because I think it would blow it,” Trump said.

-with AAP