US President Donald Trump on Thursday told reporters that the country is "down to final strokes with Iran", as the two sides engage in talks to establish an agreement designed to curb Iran's nuclear energy development in return for an easing of US sanctions.

Key to the talks has been exactly how much uranium enrichment Iran can undertake, if any at all. The Iranians say they must be able to do so for a deal to be struck. But the US does not appear to have publicly settled on a firm policy approach in that regard, given the administration's mixed messaging.

The two sides are keen on replacing the Joint Comprehensive Plan of Action, commonly known as the Obama-negotiated 2015 Iran nuclear deal that Trump withdrew from in 2018. In that deal, Iran was limited to 3.67 percent uranium enrichment, enough for civilian nuclear power and research.