In an effort to modernise its nuclear arsenal, the US has built and tested a mock bomb in a desert in Nevada.

The B61 is the nation’s main nuke. It was originally developed in the 1960s and has gone largely unchanged.

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The price tag for the bomb’s upgrade is the staggering equivalent of almost $15 billion (AUD), The New York Times reported.

That’s more than the entire cost ($12 billion) of Australia’s new squadron of Joint Strike Fighters.

A final version of the nuke was dropped at the Tonopah Test Range in Nevada on Thursday.

Meanwhile, the US is trying to get Iran to disband its nuclear program, with poor results.

US Secretary of State John Kerry has said he is prepared to “call an end” to the talks if “tough decisions” are not made.

“If the tough decisions don’t get made, we are absolutely prepared to call an end to this process,” Kerry told reporters in Vienna on Thursday.

Two deadlines to reach an accord have already been missed in the talks which have stretched into their 13th day.

“We are not going to sit at the negotiating table forever. We also recognise that we shouldn’t get up and leave simply because the clock strikes midnight,” Kerry said.

Despite progress “some of the tough issues remain unresolved,” he told reporters outside the luxury hotel in Vienna where the talks are being held.

-with AAP.