US President Donald Trump says he has instructed negotiators "not to rush into a deal" with Iran, after earlier suggesting an agreement was close.
The deal under discussion reportedly involves a 60-day ceasefire extension, reopening the Strait of Hormuz and a plan for further negotiations over Iran's nuclear programme.
In a social media post, Trump said "constructive" talks were proceeding but "both sides must take their time and get it right".
Trump said on Saturday an agreement had been "largely negotiated", prompting speculation an announcement could be imminent.
Iranian officials also signalled progress in talks over the weekend, with foreign ministry spokesperson Esmail Baghaei saying the two sides are both "very close and very far" from reaching an agreement.
According to US media, the mooted deal is not a final settlement, and instead leaves some of the thorniest issues to be negotiated later, including Iran's demand for sanctions relief, its calls for the release of frozen Iranian funds, and Washington's demands for Iran to curb its nuclear ambitions.
The reported deal has split Republicans, with some publicly arguing it is too lenient on Iran.