High-stakes talks to nail down a historic deal with Iran to curtail its nuclear program may slip past a June 30 deadline, a top US official admitted for the first time Thursday.

"We may not make June 30, but we will be close," a senior administration official said as top US diplomat John Kerry prepared to head Friday for potentially the last negotiations between Iran and global powers on the deal.

"What matters here is the substance of the deal and we have to get it right," the official said, adding that the world needed to get the right assurances that Iran's nuclear program "is entirely peaceful."

But the official said the deadline would only "slip" by a few days as they work through the "painstaking" details of what will be a "political understanding" accompanied by a series of annexes.

"The intent of everybody here -- the P5+1, the European Union, Iran -- is to stay until we get this done, or find out we can't. And our intent is to get it done," the official told reporters, asking not to be named.

Kerry is to head for the Austrian capital on Friday, with talks likely to resume on Saturday and other ministers from the so-called P5+1 group also expected to join over the weekend.

They are aiming to hammer out with Iranian Foreign Minister Mohammad Javad Zarif a complicated deal to put a nuclear bomb beyond Iran's reach.
Iranian leaders will agree to rein in their nuclear program in exchange for the lifting of a network of global economic sanctions.

Expert teams have been in Vienna for weeks trying to resolve the thorniest issues of the emerging deal, along the lines of an accord reached in the Swiss lakeside city of Lausanne on April 2.

"Everyone on the American team ... are tremendous heroes. People work from seven in the morning to midnight virtually every single day. They haven't seen their families, they haven't seen their children," the official said.

Another lingering issue is that the UN nuclear watchdog, the IAEA, must be able to satisfy long standing concerns about the possible military dimensions of Iran's nuclear program.

"Our position has never changed. We have always said that the US in particular is not looking for a confession, because we have long made our own national judgement," the official said.

"But sanctions lifting will only occur as Iran takes the nuclear steps agreed, including addressing" possible military dimensions.