Global powers negotiating with Iran to seal a deal curbing its nuclear programme are not yet where they want to be, a senior US official said Tuesday.

"We have never been closer, than we've ever been on this agreement and we are still not where we need to be to finalise a deal," a senior US administration official said.

Negotiators were taking the talks "day by day" as they seek to slot together the last pieces of a "Rubik's cube" into a complex negotiation which has lasted almost two years now.

"Sometimes things have to align in ... a moment of history to be able to do something. We are probably closer than we've ever been, because there is more of an alignment, but whether it clicks into that final cube we don't yet know," the official told reporters.

"You can get 95 percent of their way, and you can't get there in the end."

The seven nations around the negotiating table on Tuesday admitted they would not meet a deadline for a deal and gave themselves a few more days to try to reach an accord aimed at putting a nuclear bomb out of Iran's reach.

The official said there was astonishment in the US delegation at media speculation that a deal was within reach at the talks playing out behind closed doors in Vienna.

"I quite frankly think, expectations need to be based more on a sense of reality. This is very, very hard tough, stuff," the official said, asking not to be identified.

"If very tough political decisions, hard choices, can get made soon, I do believe we can get to an agreement ... it is possible," the official said, adding it would be "a tragedy" if after months of work the talks collapsed.