Donald Trump prepares to 'unleash hell' on Iran as ex-MI6 chief warns Islamic Republic has 'upper hand on United States'

She said: "President Trump does not bluff and he is prepared to unleash hell. Iran should not miscalculate again."

Ms Leavitt added: "Their last miscalculation cost them their senior leadership, their navy, their air force and their air defence system.

"Any violence beyond this point will be because the Iranian regime refused to understand."

Mr Trump launched Operation Epic Fury on February 28, later claiming the lives of Ayatollah Ali Khamenei and the Islamic Republic's security chief Ali Larijani.

Despite the US President suggesting the joint operation with Israel is now "very close" to meeting its core goals, a former MI6 chief warned Washington has underestimated Tehran.

Sir Alex Younger, who led Britain's Secret Intelligence Service from 2014 to 2020, told The Economist: "The reality is the US underestimated the task, and I think as of about two weeks ago lost the initiative to Iran."

Iran "has been more resilient" than expected by "delegating the authority" and "dispersing their military capability", Sir Alex added.

Mr Trump's attempts to bring Tehran to the negotiating table also appear to have made Iran more emboldened.

Islamic Republic commander Sayed Majid Mousavi accused the White House of an "act of war" by

He said: "Trump must learn that every threat and ultimatum against Iran is part of an act of war.

"The strikes by the fighters on the strategic points of Dimona and Haifa in recent hours, separate from pursuing Iran's military strategy, were a clear message in response to the American two-day and five-day threats."

Iran rejected a 15-point plan set out by Mr Trump earlier today, accusing the US President of putting forward "excessive" demands.

An official told Tehran state media: "Iran will end the war when it decides to do so and when its own conditions are met."

Mr Trump is demanding Tehran "never pursues nuclear weapons", pledges to dismantle nuclear facilities and hand over the enriched amounts of uranium it possesses to the International Atomic Energy Agency (IAEA).

Iran would also need to agree to limits on its missile programme, impacting both range and quantity.

Other demands include an end to Tehran funding regional proxies, including Hezbollah in Lebanon and Hamas in Gaza.

Meanwhile, Iran set out five conditions to end the conflict.

Tehran included a complete halt to "aggression and assassinations by the enemy" as part of its proposal.

Iran also wants "concrete mechanisms to ensure that the war is not reimposed on the Islamic Republic".

However, it is unclear what guarantees could be provided and which countries would take part.

Tehran also wants the payment of war damages and reparations and is calling on Israel to end attacks on its allies in the Middle East.

Washington is not expected to accept Iran's counter-proposal.