The US blockade on Iranian ships in the Strait of Hormuz would "remain in full force and effect until an agreement is reached, certified, and signed", Trump wrote on Truth Social.
"Both sides must take their time and get it right," he added.
Trump pushed back on critics who said details of a possible US-Iran agreement offer little beyond the 2015 nuclear deal negotiated by former president Barack Obama.
"If I make a deal with Iran, it will be a good and proper one ... So don't listen to the losers, who are critical about something they know nothing about," Trump said in a Truth Social post on Sunday, US time.
There was no immediate response from Iran's government. But Tasnim news agency, which is linked to Iran's Revolutionary Guards, said the US was still obstructing parts of a potential deal, including Tehran's demand for the release of frozen funds.
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.
A senior Trump administration official told reporters an agreement would not be signed on Sunday, saying the Iranian system did not move fast enough. But he outlined what he said were the latest contours of what was being negotiated.
The official, speaking on the condition of anonymity, said Iran had agreed "in principle" to open the Strait of Hormuz, in exchange for the United States lifting its naval blockade, and to dispose of Tehran's highly enriched uranium.
He said the US understood Iran's Supreme Leader Ayatollah Mojtaba Khamenei had endorsed the broad template of the deal.
There was no immediate confirmation from Iran or elaboration on what an "in principle" agreement meant.
The US official said Washington envisioned first re-opening the strait and lifting America's naval blockade. Negotiating the details of the nuclear measures would take more time, he said.
He pushed back on suggestions that Iran has not accepted disposing of its stockpiled enriched uranium. "It's a question about how," the official said.
A second senior administration official said on Sunday the proposed framework would give negotiators 60 days to reach a final deal.
Iranian sources had told Reuters that in future stages, "feasible formulas" could be found to resolve the dispute over its highly enriched uranium stockpile, including diluting the material under the supervision of the UN nuclear watchdog.
Oil prices hit two-week lows to kickstart the week with Brent crude futures down more than four per cent to $98.83 a barrel - the first time it has dipped below $100 in intraday trading since early May - while US West Texas Intermediate was at $92.03 a barrel, also down more than four per cent.
As details of the possible agreement emerged over the weekend, critics including former secretary of state Mike Pompeo and Democratic lawmakers argued it offered little beyond the 2015 Iran nuclear deal negotiated by Obama, from which Trump withdrew during his first term.
Chris Van Hollen, a Democratic member of the Senate Foreign Relations Committee, said the deal's reported outlines would amount to little more than "the pre-war status quo" with Iran.
"I think this was a blunder," Van Hollen told Fox News. "When you're digging a hole, you should stop digging, and that sounds like maybe what we're doing finally."
In another potential stumbling block, an Iranian 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.
