U.S. President Donald Trump and Iran's Supreme Leader Mojtaba Khamenei have escalated threats against each other, with Trump warning of massive military strikes and Khamenei vowing revenge for his father's death, as a temporary peace agreement between the two countries unravels.

On Saturday, Khamenei issued a written message vowing that avenging his slain predecessor and father, Ayatollah Ali Khamenei, was "the will of the nation" and "must inevitably" take place "soon." The elder Khamenei was killed in U.S.-Israeli airstrikes on February 28. In the message, the new supreme leader said "these criminals — of whom we have a complete list from top to bottom — will take with them to their graves the wish for a peaceful death in bed."

Khamenei did not explicitly name Trump in his statement, according to reporting, but crowds at his father's funeral ceremonies called for Trump's death, with mourners chanting "I swear by the blood of the supreme leader, Trump, we will kill you!" and holding placards reading "Kill Trump."

Trump responded on Friday night with a post on his social media platform Truth Social, stating that 1,000 missiles are "locked and loaded" and aimed at Iran, "with thousands of more to immediately follow" if the Iranian government assassinates or attempts to assassinate him. He said the U.S. military is ready to "completely decimate and destroy all areas of Iran" for one year, subject to extension.

The escalation comes weeks after the U.S. and Iran signed a temporary peace agreement in June. That "memorandum of understanding" called for a ceasefire, opening of the Strait of Hormuz (through which roughly 20 per cent of the world's oil passes), and a 60-day negotiation period. However, tensions have risen since, with both sides trading strikes in the Strait of Hormuz and other areas in the Persian Gulf.

Trump declared the ceasefire "over" earlier this week, calling further negotiations a "waste of time." He later contradicted himself by saying the U.S. and Iran had agreed to continue peace talks, though he repeated that the truce had ended.

Key points of contention include Iran's nuclear program and control of the Strait of Hormuz. Trump has also repeated threats to target civilian infrastructure and Kharg Island, a heavily fortified Iranian oil export hub that accounts for 90 per cent of the country's oil exports.

Mojtaba Khamenei, proclaimed supreme leader by a clerical assembly in early March following his father's death, has not been seen publicly since assuming the role. According to reporting, he suffered debilitating injuries in the strike that killed his father, with his face disfigured and limbs badly wounded. Senior sources in Tehran have said he is recovering but is not yet well enough to manage public appearances.

Sources: Forbes Business, Global News, France 24, Times of Israel