Talks to clinch a historic deal on Iran’s nuclear program may slip past a June 30 deadline, “but will be close,” a top US official admitted for the first time Thursday.

Talks to clinch a historic deal on Iran’s nuclear program may slip past a June 30 deadline, “but will be close,” 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 told reporters 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 assurance that Iran's nuclear program "is entirely peaceful."

But the official said the deadline would only "slip" by a few days.

The plan was to stay in Vienna "until we get this done, or find out we can't. But we expect to get this done," the official said, 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.