The US blockade on Iranian ships on the Strait of Hormuz would "remain in full force and effect until an agreement is reached, certified, and signed", Trump wrote on Truth Social.
Negotiations are progressing and the US relationship with Iran has become more professional and productive, he said.
"Both sides must take their time and get it right. There can be no mistakes!"
A day earlier, Trump said Washington and Iran had "largely negotiated" a memorandum of understanding on a peace deal that would reopen the Strait of Hormuz, which before the conflict carried one-fifth of global oil and liquefied natural gas shipments.
Trump has repeatedly played up the prospect of an agreement to end the war that the US and Israel started on February 28, so far without success.
It was not clear whether the agreement he was referring to on Sunday was the initial memorandum of understanding that has been under discussion, or a much more challenging broad peace settlement, likely to take much longer.
The two sides remain at odds over numerous difficult issues, such as Iran's nuclear ambitions and Tehran's demands for the lifting of sanctions and the release of tens of billions of dollars of Iranian oil revenues frozen in foreign banks.
Various media in the US and Iran had said the memorandum setting out a framework for ending months of fighting would, if concluded, lift a US blockade on Iranian shipping and reopen the waterway, which Iran has shut with threats to attack shipping.
A senior Iranian source earlier told Reuters that if Iran's Supreme National Security Council approved the memorandum, it would be sent to Supreme Leader Ayatollah Mojtaba Khamenei for final approval.
But Iran's Tasnim news agency said differences remained over one or two clauses.
Tasnim cited a source as saying there would be no final understanding if the US continued to create obstacles.
In another potential stumbling block, a military adviser to Khamenei said Tehran had the legal right to manage the Strait of Hormuz, though it was not clear if that meant continuing to decide which ships can go through.
Any deal cementing the current fragile ceasefire would bring relief to markets but not immediately quell a global energy crisis, which has driven up the costs of fuel, fertiliser, and food.
Even if the war ends now, full flows through the strait will not return before the first or second quarter of 2027, the head of the Abu Dhabi National Oil Company said last week.
Iran's Revolutionary Guards said 33 vessels had passed through the strait over the past 24 hours after getting permission from Tehran, still far short of the 140 on a typical day before the war.
Trump, while offering various war aims during the conflict, has repeatedly said the US struck Iran to prevent it from obtaining nuclear weapons.
Iran "must understand, however, that they cannot develop or procure a Nuclear Weapon or Bomb", he reiterated in his post on Sunday.
Iran has long denied it is pursuing such weapons and says it has a right to enrich uranium for civilian purposes, although the purity it has achieved far exceeds that needed for power generation.
Sources have told Reuters the proposed framework, when it emerges, will unfold in three stages: formally ending the war, resolving the crisis in the Strait of Hormuz and launching a 30-day window for negotiations on a broader agreement, which can be extended.
Trump, whose approval ratings have been hit by the war's impact on US energy prices, said on Friday he would not attend his son's wedding this weekend, citing Iran among the reasons for staying in Washington.
He spoke on Saturday with leaders from Saudi Arabia, Qatar, the United Arab Emirates, Jordan, Egypt, Turkey and Pakistan, who encouraged Trump to agree to the emerging framework, Axios reported.
