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 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 U.S. wants to a much smaller program.

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

(AP)