UN inspectors would have access to all suspect Iranian sites, including military ones, under a draft nuclear deal that six major powers and Iran are working to finalise on Tuesday, a diplomatic source said.

The person, who spoke on condition of anonymity, said the UN access would be based on consultations between the six powers and Iran under the draft deal, which would impose limits on the Iranian nuclear program in return for economic sanctions relief.

The foreign ministers of Britain, China, France, Germany, Russia and the United States met for about an hour just after midnight as they struggled to complete the agreement, which has been under negotiation for more than 20 months.

An agreement, if one can be reached, could mark a watershed in Tehran's relations with Western nations, which suspect that Iran has used its civil nuclear program as a cover to develop a nuclear weapons capability. Iran denies this.

A comprehensive meeting between Iran and the powers will be held at 0800 GMT on Tuesday, the semi-official Fars news agency reported. Iran's Foreign Minister, Mohammad Javad Zarif, and E.U.'s Foreign Policy Chief Federica Mogherini are expected to read a joint statement.

"Iran and the six powers will hold a comprehensive meeting at the UN headquarters in Vienna at 1000 am local time," Fars said.

The possible agreement, sketched out in a preliminary accord on April 2, would limit Iran's nuclear programme for more than a decade in exchange for the gradual suspension of economic sanctions that have slashed Iran's oil exports and crippled its economy.

Among the biggest sticking points in the past week has been Iran's insistence that a United Nations Security Council arms embargo and ban on its ballistic missile programme dating from 2006 be lifted immediately if an agreement is reached.

Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.

Western nations are loathe to allow Iran to buy and sell arms freely, fearing this would permit it to increase its military support to Shi'ite militias in Iraq, Houthi militants in Yemen and embattled President Bashar al-Assad in Syria.

Another key stumbling block has been the so-called "snapback" plan to restore the sanctions if Iran violates the deal. It was not clear how those issues would be finessed in any final agreement, although diplomats said they were close to overcoming differences.

Other problematic issues include access for inspectors to military sites in Iran, explanations from Tehran of past activity that might have been aimed at developing a nuclear weapon and the overall speed of sanctions relief.

The diplomatic source said that if the deal is agreed, a UN Security Council resolution on it would ideally be adopted in July and steps to be taken by both sides - including Iranian limitations on its nuclear programme and relief from sanctions on Iran - would be implemented in the first half of 2016.

The information from the source was preliminary and subject to change because it was based on a draft of the nuclear deal that was not the final version and that could be amended before final approval by Iran and the six powers.

The source said Iran and the UN International Atomic Energy Agency have agreed on a plan to address outstanding questions about the possible military dimensions of past Iranian nuclear activity by the end of 2015, adding that some sanctions relief would be conditioned on Tehran resolving this issue.

The plan agreed by the IAEA and Iran includes one visit to the Parchin military site as well as possible interviews with Iranian nuclear scientists, the source noted.

The marathon nuclear negotiations, which have been going for more than two weeks in Vienna, missed a midnight deadline on Monday to reach a final deal, but diplomats from all sides said they hoped for a breakthrough in the coming hours. – Reuters