Dubai (AP): Iran's underground enrichment site at Fordo was hit again 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 Fordo 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 Fordo facility already following the Sunday's US airstrike there with sophisticated bunker-buster bombs.

“Given the explosive payload utilized ... 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 US 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. One Israeli strike hit the gate of Iran's notorious Evin prison in Tehran, Iranian state television reported.

The report shared what appeared to be black-and-white-surveillance footage of the strike. The prison is known for holding dual nationals and Westerners often used by Iran as bargaining chips in negotiations with the West.

Israel did not immediately acknowledge carrying out the strike.

Evin also has specialized units for political prisoners and those with Western ties, run by the paramilitary Revolutionary Guard, which answers only to Supreme Leader Ayatollah Ali Khamenei. The facility is the target of both U.S. and European Union sanctions.

Earlier Monday, Iranian Gen. Abdolrahim Mousavi, the chief of joint staff of armed forces, warned Washington its strikes had given Iranian forces a “free hand “ to "act against US interests and its army.”

Tens of thousands of American troops are based in the Middle East, many in locations within range of short-range Iranian missiles.

The US described its Sunday attack on the Fordo and Natanz enrichment facilities, as well as the Isfahan nuclear sit, as a one-off to take out Iran's nuclear programme, but President Donald Trump has warned of additional strikes if Tehran retaliates.

Mousavi described the American attack as violating Iran's sovereignty and being tantamount to invading the country, the state-run IRNA news agency reported.

In the wake of the American attacks on Iran, calls came from across the globe for de-escalation and the return to diplomacy to try and resolve the conflict.

On Monday, the European Union's top diplomat said the bloc remained “very much focused on the diplomatic solution.”

“The concerns of retaliation and this war escalating are huge,” Kaja Kallas said at the start of a foreign ministers' meeting in Brussels where Iran has jumped to the top of the agenda.

“Especially closing of the Strait of Hormuz by Iran is something that would be extremely dangerous and not good for anybody,” Kallas said, referring to a maritime route crucial for oil transport.

After Sunday's attacks, Iranian officials repeated their longtime threats of possibly closing the key shipping lane.

Iran, which insists its nuclear programme is for civilian purposes only, previously agreed to limit its uranium enrichment and allow international inspectors access to its nuclear sites under a 2015 deal with the US, France, China, Russia, Britain and Germany in exchange for sanctions relief.

But after Trump pulled the US unilaterally out of the deal during his first term, Iran began enriching uranium up to 60% — a short, technical step away from weapons-grade levels of 90% — and restricting access to its nuclear facilities.

As he arrived in Brussels on Monday for a meeting with his EU counterparts, German Foreign Minister renewed calls for Iran to agree again to direct talks with the United States but says Europe still has a role to play.

“We already made it very clear to the Iranian side that a real precondition for a settlement to the conflict is that Iran be ready to negotiate directly with the US,” he said, while adding that the European group known as the E3 “will contribute what we can.”

Iranian Foreign Minister Abbas Araghchi was meeting on Monday in Moscow with Russian President Vladimir Putin, one of Iran's key allies.