ISTANBUL/TEL AVIV (Reuters) -Israel struck the most notorious jail for political prisoners in Tehran on Monday in a potent demonstration that it was expanding its targets beyond military and nuclear sites to aim squarely at the pillars of Iran's ruling system.

Iran repeated earlier threats to retaliate against the U.S. But it had yet to do so in a meaningful way more than 24 hours after U.S. bombers dropped 30,000-pound bunker-busters on Iran's underground nuclear sites, while President Donald Trump openly mused about overthrowing the Iranian government.

Two U.S. officials said Washington assessed that Iran could carry out attacks targeting American forces in the Middle East soon, although the U.S. is still seeking a diplomatic resolution that would see Tehran forgo any reprisal.

In Qatar, home to the biggest U.S. military base in the Middle East, the U.S. embassy advised Americans to shelter in place, out of what it said was "an abundance of caution".

Despite Iran's threats to challenge oil shipments from the Gulf, prices largely held steady, suggesting traders doubted the Islamic Republic would follow through on any action that would disrupt global supplies.

Iranian Foreign Minister Abbas Araqchi met Russian President Vladimir Putin in Moscow as Tehran sought backing from one of its last major power friends for its next steps.

'VIVA LA LIBERTAD'

Israel made clear that its strikes on Evin prison and other targets in Tehran were intended to hit the Iranian ruling apparatus broadly, and its ability to sustain power.

"Viva la libertad!" Foreign Minister Gideon Saar wrote on X, accompanying a video of an explosion at a building with a sign identifying it as an entrance to Evin prison.

Reuters could not immediately verify the video posted by Saar, although other videos showing the aftermath of the strike on the prison were verified as genuine.

Iran's IRIB state broadcaster released video showing rescue workers combing the flattened wreckage of a building at the prison, carrying a wounded man on a stretcher. The Mizan news outlet of Iran's judiciary said urgent action was being taken to protect the health and safety of inmates there.

Evin has long been Iran's primary prison for housing political detainees and security prisoners, as well as the site of executions that remain strong memories for the opposition. Several high-profile foreign prisoners are also held there.

Israel's military said it had also struck Revolutionary Guard command centres responsible for internal security in the Tehran area.

The military was "currently striking, with unprecedented force, regime targets and governmental repression bodies in the heart of Tehran," Defence Minister Israel Katz said in a statement.

Much of Tehran's 10 million population has fled after 10 days of bombing. Tasnim news agency reported a strike at an electricity feeder station in the Evin neighbourhood. Power company Tavanir reported some areas in the capital saw electricity cuts.

LIMITED OPTIONS

Since Trump joined Israel's campaign by dropping massive bunker-buster bombs on Iranian nuclear production sites on Sunday morning, Iran has repeatedly threatened to retaliate.

"Mr Trump, the gambler, you may start this war, but we will be the ones to end it," Ebrahim Zolfaqari, spokesperson for Iran's Khatam al-Anbiya central military headquarters, said on Monday in a recorded, English-language video statement.

Trump's administration maintains that its aim is solely to destroy Iran's nuclear programme, not to open a wider war. But in a social media post on Sunday, Trump spoke of toppling the hardline clerical rulers who have been Washington's principal foes in the Middle East since Iran's 1979 Islamic Revolution.

"It's not politically correct to use the term, 'Regime Change,' but if the current Iranian Regime is unable to MAKE IRAN GREAT AGAIN, why wouldn't there be a Regime change??? MIGA!!!" he wrote.

Five insiders with knowledge of the discussions said efforts had been ramped up to anoint a successor for Iran's 86-year-old supreme leader, Ayatollah Ali Khamenei. His son Mojtaba, 56, and Hassan Khomeini, 53, grandson of the revolution's founder Ayatollah Ruhollah Khomeini, are now seen as the frontrunners, they said.

The younger Khomeini, a longstanding ally of the reformist faction that has largely been sidelined by hardliners in recent years, has entered the frame as a serious candidate this month as a potentially conciliatory choice, the five people said. His lineage means he also commands respect among hardliners.

MORE ISRAELI STRIKES

Israel's airstrikes on Iran have met little resistance from Iranian defences since Israel launched its surprise attack on June 13, killing many of Iran's top commanders.

Iranian officials said last week that more than 400 people had been killed in the Israeli attacks, mostly civilians. Israel says Iran's retaliatory missile strikes killed 24 people and injured hundreds, the first time a significant number of Iranian missiles have ever penetrated Israeli defences.

The Israeli military said a missile launched from Iran early on Monday had been intercepted by Israeli defences. Air raid sirens blared overnight in Tel Aviv and other parts of central Israel.

But Iran's ability to hit back is far more limited than a few months ago, since Israel inflicted defeat on Tehran's most feared regional proxy force, Hezbollah in Lebanon. Iran's most powerful client ruler, Syria's Bashar al-Assad, fell soon after.

Iran's most effective threat to hurt the West would be to restrict global oil flows from the Gulf, but oil prices have not shot up to crisis levels. After briefly jumping above $80 a barrel at the open, Brent crude futures were down 22 cents, or 0.29%, at $76.79 a barrel in noon trading. [O/R]

Iranian lawmakers have discussed shutting the Strait of Hormuz that leads into the Gulf, though no decision has been taken.

"It's economic suicide for them if they do it. And we retain options to deal with that," U.S. Secretary of State Marco Rubio said.

(Reporting by Reuters; writing by Peter Graff; editing by Bernadette Baum and Mark Heinrich)

By Parisa Hafezi and Alexander Cornwell