Diplomats say Iran is on course for its stockpile of enriched uranium to breach the 2015 nuclear agreement within days.
Iran had set Thursday as a deadline beyond which it would exceed the limits allowed under its nuclear deal with major powers, which the US abandoned it last year.
However, President Trump said on Friday: "We have a lot of time. There's no rush."
US President Donald Trump this week. Source: AAP
"They can take their time. There's absolutely no time pressure. I think in the end, hopefully, it's going to work out. If it does, great - and if it doesn't, you'll be hearing about it," he said on the sidelines of a G20 summit in Osaka.
Other world leaders gathered in Japan continued to express concern about Iran, even as Trump appeared relaxed.
Chinese President Xi Jinping said the Gulf region was "standing at a crossroads of war and peace", calling for calm and restraint and talks to resolve the issue.
European Council President Donald Tusk, also at the G20, said the European Union would continue to monitor Tehran's compliance.
"Maintaining the nuclear deal is in the regional and international security interest," Tusk said. "The EU is committed to the deal as long as Iran continues to uphold it."
The United States withdrew last year from the pact, under which Iran accepted curbs on its nuclear program in return for relief from economic sanctions. Iran has said it wants to abide by the agreement but cannot do so indefinitely because new US sanctions mean it is receiving none of its benefits.
US-Iranian military tensions have risen over the past two months. Washington accused Iran of carrying out attacks on six tankers in May and June, which Tehran denies.
Iran shot down a US drone last week it said was in its airspace. The United States said it was in international skies.
Trump said later he hoped to avoid war but that, if one took place, it would be short and not involve boots on the ground. Iranian Foreign Minister Mohammad Javad Zarif said on Twitter in response on Thursday a "short war with Iran is an illusion".
