The United States denied Wednesday making concessions to let Iran off the hook by not declaring any past military activities relating to its suspect nuclear program.

Six world powers are leading negotiations to reach a deal to put an atomic bomb beyond Iran's reach, with tensions rising as a June 30 deadline looms.

One of the major stumbling blocks has been winning access to Iran's military sites where Iranian scientists may have developed nuclear arms in the past.

Despite Iran's assertions that its nuclear program is purely peaceful and for civilian energy purposes only, the UN watchdog, the IAEA, has for years said it cannot certify that is true.

Top US diplomat John Kerry told reporters on Tuesday that negotiators were "not fixated on Iran specifically accounting for what they did at one point in time or another. We know what they did. We have no doubt."

His spokesman, John Kirby, stressed on Wednesday that Kerry's words should not be interpreted as dropping a long-held insistence that Iranian leaders account for past programs.

"The IAEA's concerns about possible military dimensions past and present, have to be fully addressed before there's going to be a deal," Kirby told reporters.

Kerry "didn't say, and we've never said, that past potential military dimensions ... don't matter. Of course, they matter. We wouldn't be sitting down with them having a negotiation about this if it didn't matter," Kirby added.

Chief US negotiator Wendy Sherman was back in Vienna Wednesday for new talks with her counterparts seeking to pin down a deal.

Despite recovering from breaking his right leg late last month, Kerry has said that he is preparing to join the negotiations ahead of the June 30 deadline, although no date has yet been set for his travel.