Iran's underground enrichment site at Fordow was hit again on Monday while Iran fired a salvo of missiles and drones at Israel and warned the United States that its military now has a “free hand” to attack American targets in the wake of the Trump administration's massive strikes on Iranian nuclear sites.

The Fordow facility was one of those hit in Sunday's attack, and it was struck again on Monday, Iranian state television reported.

There was no immediate word on damage nor who launched the attack, though Israel said earlier it was conducting airstrikes on Iran.

In Vienna, the head of the United Nations nuclear watchdog said he expected there to be heavy damage at the Fordow facility already following the Sunday's US airstrike with sophisticated bunker-buster bombs.

“Given the explosive payload utilised... very significant damage... is expected to have occurred,” said Rafael Grossi, the head of the International Atomic Energy Agency.

With the strikes on Sunday on Iranian nuclear sites, the United States inserted itself into Israel’s war, prompting fears of a wider regional conflict.

Iran said the United States had crossed “a very big red line” with its risky gambit to strike the three sites with missiles and 30,000-pound bunker-buster bombs .

Israel said its defence systems were operating to intercept the latest Iranian threat, which apparently targeted the north and central areas of the country, and told people to head to shelters.

Iran described the attack a new wave of its Operation “True Promise 3,” saying it was targeting the Israeli cities of Haifa and Tel Aviv, according to Iranian state television.

Explosions were also heard in Jerusalem. There were no immediate reports of damage.

In Iran, witnesses reported Israeli airstrikes hit areas around Iran’s capital, Tehran, around midday.

They hit a power supply system in the Iranian capital, triggering outages in some areas around the city, Iranian media reported.

The power distribution line in northern Tehran "was damaged, causing outages in some areas", Fars news agency reported. (AP)