Iran and six world powers began a new round of negotiations on Tuesday aimed at settling the dispute over Tehran's nuclear programme by late July, despite wide differences on how to get there.

Chief negotiators from Iran, the US, France, Germany, Britain, China and Russia started a two-day meeting around 9:45 am (0745 GMT) at the UN complex in Vienna, where they have held two previous such sessions since February.

Both sides say they want to start drafting a comprehensive agreement in May, some two months before a July 20 deadline for finalising the accord. Western official say, however, that the two sides are still far apart on key issues.

The powers want Iran to scale back its nuclear programme to deny it any capability to quickly make an atomic bomb, if it decides to build one. Iran says its nuclear programme is entirely peaceful and wants them to lift sanctions.

"We're still in an exploratory phase," one diplomat from the powers said.

The six nations have agreed internally to have a text by the end of May or early June, the diplomat said, adding however: "In the end, things will happen in July."

The negotiations can be extended by another half-year if both sides agree to do so, and negotiate the content an extension deal.

Tuesday's opening session was chaired by Iranian Foreign Minister Mohammad Javad Zarif and European Union foreign policy chief Catherine Ashton, who coordinates the talks on behalf of the six powers.

Iran says its enrichment programme is a peaceful bid to generate electricity and has ruled out shutting any of its nuclear facilities.

But the US and some other Western countries have accused it of working on developing a nuclear bomb capability. Israel has threatened to attack its long-time foe Iran if diplomatic efforts fail. Iran says it is Israel's assumed atomic arsenal that threatens peace and stability in the Middle East.

The diplomat said issues to be discussed at the April 8-9 meeting included how the United Nations's nuclear watchdog would verify whether Iran was meeting its end of any deal, suspected past atomic bomb research by Tehran, and how to deal with UN Security Council resolutions on Iran adopted since 2006.

A senior Iranian negotiator, Deputy Foreign Minister Abbas Araqchi, said major issues discussed in previous meetings - Iran's uranium enrichment and a heavy-water reactor at Arak - would also be debated.

The aim of the negotiations begun almost two months ago is to hammer out a long-term deal to define the permissible scope of Iran's nuclear programme in return for an end to sanctions.

In November, Iran and the six nations agreed an interim accord to curb Tehran's atomic activities in exchange for some easing of sanctions. The six-month deal, which took effect on January 20, was designed to buy time for talks on a final accord. - Reuters