US Secretary of State John Kerry is set to launch a final high-stakes diplomatic push to seal a ground-breaking nuclear deal with Iran, as some officials warned Wednesday the negotiations may go beyond a June 30 deadline.

Kerry will on Saturday once again huddle with his Iranian counterpart Mohammad Javad Zarif in Geneva, after weeks of behind-the-scenes complex technical discussions in Vienna seeking to narrow the gaps on what would be an unprecedented deal on curtailing Iran's nuclear program.

Iran and the six global powers leading the talks -- Britain, China, France, Germany, Russia and the United States -- laid down a framework to guide the final accord in eight days of marathon late-night talks in Lausanne in early April.

Kerry and his team will now return to Europe for what is expected to be a final series of meetings with Zarif as the clock ticks down to June 30 and a possible deal putting a nuclear bomb out of Iran's reach.

The deal will include Iran dramatically scaling back its nuclear activities and submitting those that remain to what US President Barack Obama has called the "most robust and intrusive inspections and transparency regime ever negotiated."

In return, the United States and the five other major powers committed to lift certain sanctions that have caused the Islamic republic of 75 million people major economic pain.

A deal would draw the curtain on a crisis that has raged since Iran's nuclear activities were first revealed some 12 years ago.

But Iranian negotiator Abbas Araghchi, quoted by state news agency IRNA, said Wednesday the two sides were "not bound by the schedule" agreed on April 2.

"We are not at the point where we can say that negotiations will be completed quickly -- they will continue until the deadline and could continue beyond that," he said.

A State Department official, Jeff Rathke, said however the US was "focused on June 30th as the deadline, and that's what we're devoting our efforts to."

"We won't have a deal until those technical details are done, and we expect the pace of the talks to continue unabated," he told reporters, adding "we think we can achieve that goal."

On Monday, deputy oil minister Amirhossein Zamani-Nia was quoted as saying that 20 pages of the text had been written "but there are still disagreements and 30 percent of the work remains to be done."

- Military concerns -

One of the major sticking points appears to be access to military sites amid lingering concerns about the possible military dimensions of the Islamic Republic's nuclear program.

Tehran has always denied seeking to develop a nuclear bomb, saying its nuclear energy program is for civilian purposes only.

But French Foreign Minister Laurent Fabius warned on Wednesday that France would oppose a final nuclear accord unless it allowed inspections of military sites.

An agreement "will not be accepted by France if it is not clear that verifications can be made at all Iranian facilities, including military sites," Fabius told parliament.

On Tuesday, Yukiya Amano, the head of the UN's atomic watchdog, said Iran had agreed to implementing the Additional Protocol of the Nuclear Non-Proliferation Treaty (NPT) that allows for snap inspections.

"When we find inconsistency or when we have doubts, we can request access to the undeclared location for example, and this could include military sites," the Japanese diplomat told AFP.

Zarif has said the April framework allows "some access" but not inspections of military sites, in order to protect national "military or economic secrets."

Another hurdle is how to lift an interlocking network of biting US, EU and UN sanctions, with global powers insisting that these should be able to snap back into place should Tehran violate the deal.

Both Iran and the United States are also under intense pressure domestically from hardliners at home who view the other side with suspicion.

Iran's supreme leader Ayatollah Ali Khamenei praised the work of his nuclear negotiators Wednesday after they came under heavy criticism from conservatives in parliament.

The negotiators "are working, making efforts, breaking sweat... to obtain what is in the interests of the country and the regime", he told members of parliament.