Donald Trump has declared that the Strait of Hormuz will be “completely open” from Friday, as Western leaders gathering at the G7 summit in Évian-les-Bains battled to prevent the fragile US deal with Iran from almost immediately unravelling.

“The deal’s all signed. And the strait is already partially opened,” Trump said as he arrived at the summit in France, but Israeli breaches of the ceasefire in Lebanon and Iran’s claims about its right to charge fees in the crucial waterway revealed the agreement’s many loose ends.

Speaking at the start of bilateral talks with the French president, Emmanuel Macron, on Monday, Trump rejected a proposed UK-France joint naval mission in the strait, saying “I don’t think we will need much help” keeping it open.

“I think a lot of great things are going to happen in the Middle East right now. And very importantly, the oil is plummeting down and the stock market is shooting up like a rocket today,” Trump said. “The main thing is that Iran will not have a nuclear weapon. They fully agreed to that with strong policing powers, and they won’t have a nuclear weapon, which is what it was all about.”

The memorandum of understanding – which US officials said would open the Strait of Hormuz in exchange for a lifting of a US naval blockade on Iran – is set to be formally signed at a ceremony in Geneva on Friday attended by the US vice-president, JD Vance, and the chief Iranian negotiator, Mohammad-Bagher Ghalibaf.

White House officials said the full details of the agreement would be published in the next 24 to 48 hours.

But the G7 leaders gathering for three days of talks found themselves already trying to shore up the agreement that the US had signed.

Technical discussions led by Vance from the US side will begin later this week, including the more thorny issues of the fate of Iran’s nuclear programme, which Trump has declared must never be able to produce a nuclear weapon.