* Iran, world powers to meet in Vienna on April 8-9
* Tehran says nuclear work peaceful, denies bomb accusations
* Both sides say drafting of agreement could start in May
The Islamic Republic and six world powers will hold a new round of talks in Vienna on Tuesday and Wednesday intended to reach a comprehensive agreement by July 20 on how to resolve a decade-old standoff that has stirred fears of a Middle East war.
It will be the third meeting of chief negotiators since February. So far, officials say, they have largely focused on what issues should form part of a long-term deal.
"We will finish all discussions and issues this time to pave the ground for starting to draft the final draft in Ordibehesht (an Iranian month that begins in two weeks)," Iran's Foreign Minister Mohammad Javad Zarif said upon arrival in Vienna.
A U.S. official gave a similar timetable last week, voicing hope that the drafting of an agreement could begin in May.
Iran says its enrichment programme is a peaceful bid to generate electricity and has ruled out shutting any of its nuclear facilities.
But the United States 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.
The relatively upbeat comment by Zarif appeared designed to underline Tehran's commitment to reach a comprehensive deal by the July deadline, though Western officials say wide differences remain between the two sides.
A spokesman for European Union foreign policy chief Catherine Ashton, who coordinates contacts with Iran on behalf of the powers, said the discussions would be "detailed and substantial" but gave no details.
"The next round of talks will be an important continuation to explore respective positions on each topic," the spokesman, Michael Mann, said.
The six powers - United States, France, Russia, China, Britain and Germany - want Iran to scale back its nuclear programme so it cannot quickly make a nuclear bomb, if it decided to pursue such arms. Iran wants the six powers to lift sanctions that are severely hurting its oil-dependent economy.
TALKS STILL IN EARLY STAGES - RUSSIA
Iran says the powers must respect what it calls its right to a peaceful nuclear programme, including the enrichment of uranium. Such activity can have both civilian and military uses.
"We believe that our partners should make important decisions which includes respecting the existing realities and respecting Iran's rights," Zarif said.
"We are ready to cooperate to remove any ambiguity about the peaceful nature of our nuclear programme."
A senior U.S. administration official, speaking on Friday, said both sides intended to spend March and April going over "every single issue that we believed had to be addressed in a comprehensive agreement" before work started on drafting in May.
"We are on pace with that work plan, looking toward beginning drafting in May," the official said.
Russia's chief negotiator said Moscow had no special expectations for this week's meeting. Deputy Foreign Minister Sergei Ryabkov said talks on a number of issues were still in early stages and the meeting should produce a basis for further discussions, Itar-Tass news agency reported.
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 Jan. 20, was designed to buy time for talks on a final accord.
Iran has said it had useful expert-level nuclear talks with world powers in Vienna last week, addressing all major technical issues in the way of a final settlement.