A camera man stand in front of a poster of the Iran talks where closed-door nuclear talks take place at the International Centerin Vienna, Austria, Wednesday, June 18, 2014. (AP Photo/Ronald Zak)

Vienna -

An Iranian official says some progress is being made at nuclear talks with six world powers ahead of a July 20 target date for a deal, but “substantial differences” remain.

Foreign Ministry spokesman Marzieh Afkham spoke on Wednesday as negotiators worked on a draft agreement meant to curb Iran's nuclear program in exchange for an end to sanctions.

Diplomats familiar with the closed-door talks say the main problem remains uranium enrichment, which can make both reactor fuel and the core of a nuclear weapon.

Iran denies wanting nuclear arms but insists on having either 50 000 enriching centrifuges, or fewer but more advanced machines with the same total output. The US wants to a much smaller program.

Russia, China, Britain, France and Germany are also at the Vienna talks. -