Here are the key sticking points:

- Iran's hold on Hormuz -

Iran blockaded the Strait of Hormuz in retaliation for the war launched by the United States and Israel on February 28, and its two-month chokehold on the strategic trade waterway has battered the global economy.

Iran has also laid mines in the strait and put forward plans to charge transit fees, an idea rejected by Washington and the UN maritime agency.

Tehran said on April 17 it would reopen the waterway to commercial vessels, only to reverse its position after US President Donald Trump said he would maintain a naval blockade of the country's ports.

Iran has since reportedly proposed easing its closure of the strait if Washington lifts its retaliatory blockade while broader negotiations carry on, including over the country's nuclear programme.

But Trump is unlikely to accept the proposal, according to reports.

- US blockade of Iranian ports -

Trump ordered the blockade of Iranian ports after talks in Islamabad failed to reach a deal after an April 8 ceasefire announcement.

Tehran condemned the move as an "act of war" and a ceasefire violation.

Iran's economy is suffering from the blockade as well as the closure of the strait, with the rial falling to historic lows against the dollar.

The Wall Street Journal, citing unnamed officials, reported Trump told US national security officials to prepare for a long blockade of Iran's ports.

He said the US Navy would continue to squeeze Iran's key oil exports until Tehran agrees to all of Washington's demands, the newspaper reported.

Trump, however, is also under domestic pressure to find an off-ramp as fuel prices rise, with midterm elections due in November and polls showing the war is unpopular among Americans.

- Iran's nuclear programme -

The United States and Israel launched strikes on Iran in the midst of negotiations in which Washington demanded Tehran hand over its stockpile of enriched uranium and stop any further enrichment.

Trump said at the beginning of the war that Tehran had "attempted to rebuild their nuclear programme", an assertion not backed by the United Nations nuclear watchdog.

"They've rejected every opportunity to renounce their nuclear ambitions. And we can't take it anymore," he said in his address announcing the US-Israeli attack in late February.

But Iran has denied it is seeking a nuclear weapon and insists its programme is solely for civilian purposes.

Iran has proposed shelving nuclear discussions to prioritise talks on the strait.

But seizing the country's stockpile of enriched uranium has been a key objective of the war for Trump -- who pulled out of a deal on Iran's nuclear programme in 2018, reinstating crippling sanctions.

Trump said on Tuesday the United States would "never let that opponent (Iran) have a nuclear weapon", saying the Islamic republic had been "militarily defeated".

Iran officials for their part have voiced defiance, saying the Islamic republic is stable and remains powerful.

Iran's top diplomat Abbas Araghchi blamed Washington on Monday for the failure of peace talks during a visit to Russia, citing "excessive demands".

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