"It's a memorandum of understanding. And if I don't like it, we'll go back to shooting at them, dropping bombs on their head," Trump said on Wednesday at a G7 summit in France.
"If I don't like it, if they don't behave, we'll go right back to dropping bombs right smack in the middle of their head, OK?"
Leaders of the G7 countries demanded a ceasefire in Lebanon on Wednesday and said they will diversify energy supply routes to reduce dependence on the Strait of Hormuz in response to the war in Iran, as they welcomed the interim deal to end it.
The leaders met for a summit in the French town of Evian-les-Bains, an hour's drive along the shore of Lake Geneva from where the memorandum is due to be signed at a ceremony across the Swiss border on Friday.
The US-Iran agreement is expected to launch negotiations towards a final settlement to end the war, which has killed more than 7000 people, mostly in Iran and Lebanon.
"We underline the need for the negotiation ... to address the threats posed by Iran in the region and beyond and ensure that they never obtain a nuclear weapon," the leaders said in a statement.
The summit gave Trump a chance to present his deal with Iran to allies Britain, Canada, France, Germany, Italy and Japan.
They share Washington's concerns about Iran's nuclear program and other issues, but never endorsed his decision to go to war, and worry Tehran has gained leverage by withstanding the superpower onslaught and asserting control over the strait.
The leaders said they were ready to contribute to the implementation of the accord, with a coalition led by Britain and France set to help secure shipping once the Strait of Hormuz reopens.
The memorandum of understanding signed by Washington and Tehran on Monday, though yet to be made public, extends a ceasefire announced in April by another 60 days to allow them to negotiate a permanent truce.
Trump appears to have achieved little of what he said he wanted at the outset of the war.
Iran's theocratic government remains in place, its stockpile of highly enriched uranium has not been surrendered, its ballistic missile capabilities have not been destroyed and it has not ended its support for anti-Israel militias such as Hezbollah in Lebanon.
Trump says the agreement states that Iran will not have a nuclear weapon - a restatement of Iran's official position since the 1970s.
US officials say further discussions will lead to the removal or destruction of its enriched uranium stockpile.
But ending the war on such terms could still expose Trump to criticism, including from hawks within his own Republican party, ahead of midterm elections in November.
One of the biggest questions still hanging over the truce is the fate of Lebanon, which Israel invaded in March to root out Hezbollah after the militant group fired across the border in solidarity with Tehran following US-Israeli strikes on Iran.
Israeli forces still occupy a swathe of southern Lebanon, where more than a million people have been driven from their homes, while Hezbollah remains undefeated.
Iran says the ceasefire must also end hostilities in Lebanon, and that a permanent deal must lead to an Israeli withdrawal.
Israel, which was excluded from the US-Iran peace negotiations, says it will not withdraw and reserves the right to use military force.
That has opened up a rift between Israel and the United States, with Trump publicly berating his wartime ally Israeli Prime Minister Benjamin Netanyahu.
In their statement, the G7 leaders called for an "immediate robust ceasefire" in Lebanon and the disarmament of Hezbollah.
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