Nuclear talks between Iran and six world powers moved to an ambitious new stage on Wednesday, with the two sides sitting down to start writing the text of a final deal. But with major issues still unresolved, any initial draft is likely to be a patchwork affair — and agreement remains uncertain.

The talks are being coordinated by EU foreign policy chief Catherine Ashton, and her spokesman, Michael Mann, said the two sides are “getting down to the nitty-gritty” in discussions scheduled to adjourn Friday, about two months before the July 20 target date for a deal.

The United States and its allies hope to reduce Iran’s potential nuclear weapons-making capacity by negotiating substantial cuts in its atomic program. Tehran says it has no interest in such weapons but is ready for some concessions if all sanctions on its economy are lifted.

Two diplomats involved with international efforts to trace and curb Iran’s atomic activities said the two sides were coming to the table with some differences narrowed but others remaining.

The diplomats demanded anonymity because they are not authorized to discuss the confidential talks. But they gave a partial picture of where things stand.

Areas of progress include:

ARAK

The partially built reactor at Arak was meant to be a heavy-water facility that would produce substantial amounts of waste plutonium — material that can be used as the core of a nuclear weapon. There is tentative agreement on re-engineering the reactor to a light-water installation or cutting back on its output.

MONITORING A DEAL

Iran is ready in principle to sign an agreement with the U.N. atomic agency that would allow its experts to visit any declared nuclear site at very short notice; investigate suspicions of undeclared nuclear activity, and push for deeper insight into all atomic work.

Major differences remain on:

URANIUM ENRICHMENT

Iran now has nearly 20,000 centrifuges set up, with about half of them producing uranium enriched to reactor fuel-grade levels. Iran says it is enriching only for peaceful purposes but if reconfigured, the centrifuges could produce weapons-grade uranium for nuclear bombs.

The United States, Britain, France and Germany say no more than a few thousand of the machines should be left standing. Tehran wants to expand the program — or at least keep the status quo.

Russia is ambivalent about numbers, as long as Iran agrees to allow the U.N. nuclear agency greater monitoring and investigating authority, while China normally supports Russia’s position.

MISSILES

The United States and its allies say Iranian missiles capable of delivering a nuclear weapon must be dealt with in any deal. But Iranian Supreme Leader Ayatollah Ali Khamenei said earlier this week that hopes of including the missiles are “a stupid, idiotic expectation.” One of the diplomats said possible compromises could focus on nuclear missile payloads, or limiting further test launches.

THE NUCLEAR PAST

The United States and its allies allege that Iran worked on nuclear weapons in the past and say Tehran must admit it to secure a nuclear deal. Tehran says such accusations are baseless.

A U.N. probe into the accusations relaunched three months ago has so far not narrowed differences.

DURATION

The U.S. and its allies want to limit the scope of Iran’s nuclear program for more than a decade. Tehran wants any constraints lifted after only a few years.

(AP)