The United States says it will on Monday begin trying to free merchant ships stranded by Iran's closure of the key shipping channel, the Strait of Hormuz, since the start of the war,
BBC reports.
US President Donald Trump said "Project Freedom" was a "humanitarian gesture" and that any interference would "be dealt with forcefully".
Some 15,000 US service personnel, guided-missile destroyers and more than 100 aircraft would be involved, US Central Command (Centcom) said, adding that the US blockade of Iranian ports would continue.
The move comes as the two countries observe a temporary ceasefire, which began on 8 April, and as they work on agreeing on a permanent peace plan.
However a senior Iranian official warned that any US attempt to interfere in the strait would be considered a violation of the ceasefire,
Iran's military said it will attack any foreign forces if they enter the strait, "especially, the aggressive US army".
Iran's closure of the channel - which is crucial to oil exports from the Gulf - has seen fuel prices rise globally and also left thousands of sailors stranded on some 2,000 ships trapped by the blockade.
About 20% of the world's oil and liquefied natural gas usually passes through the strait.
In Trump's post on social media, he said "countries from all over the world" had asked the US if they could help free up the ships, describing them as "merely neutral and innocent bystanders".
Trump added the operation will be made on behalf of the US, Iran and other Middle Eastern nations, without listing which countries. He did not provide further details on how the operation would be managed.
He also said that US representatives were having "very positive" discussions with Iran, and that those talks "could lead to something very positive for all".
An estimated 20,000 sailors have been trapped in the Gulf since the start of the war with Iran. There has been growing concern over dwindling supplies and the effects on sailors' physical and mental health.
The United Kingdom Maritime Transportation Operation (UKMTO) on Sunday reported that a tanker had been hit by an "unknown projectile" in the strait, adding that the crew were safe.
Also on Monday, the Pakistani government said 22 crew members from Iranian container ship Touska - seized by the US last month - will be "handed over to the Iranian authorities".
The Touska was intercepted on its way to an Iranian port as part of the US's blockade, with the crew evacuated to Pakistan.
Pakistan said the repatriation of the crew is "a confidence-building measure by the United States of America".
