US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif met for six-hour talks on Saturday aimed at resolving the remaining differences over the Iran nuclear deal.

Just weeks ahead of the June 30 deadline for a final and comprehensive agreement to settle the 12-year standoff on the Iran nuclear problem, a diplomatic source said there are still some issues and that the talks on Saturday ended without any big breakthrough.

Asked about completing the full accord by June 30, Zarif said, "We will try."

According to a source from the Iranian delegation, talks will resume next week in Vienna at the level of deputies and experts.

The world's major powers and Iran reached a first-step agreement on Iran's nuclear program on Nov 24, 2013, which required Iran to suspend some sensitive nuclear activities in exchange for limited sanction relief to buy time for diplomatic efforts to resolve the issue.

On April 2, they reached a framework understanding on major disputes, however problems remained.

Earlier on Saturday, Iran's senior nuclear negotiator Abbas Araqchi said some enduring differences between Iran and the West over the nuclear program have hindered the progress in preparing the draft for a possible nuclear agreement.

At the meeting on Saturday, all the issues were reviewed, but the differences still remain, Araqchi was quoted as saying by state IRNA News Agency.

Earlier in the day, Araqchi said that one of the subjects in the talks between Kerry and Zarif would be the details and implementation of the additional protocol to the Non-Proliferation Treaty.

Iran has emphasized that it would not allow International Atomic Energy Agency inspectors to have access to its military sites under the additional protocol.

The next round of nuclear talks between the representatives of Iran and the five members of the UN Security Council - the United States, Britain, France, Russia, China - plus Germany will be held on Thursday in Vienna, Araqchi said on Saturday.

Negotiators have been divided in limiting Iran's uranium enrichment capacity and the steps to lift sanctions that are crippling the Iranian economy.

Xinhua - AP