Top officials from the United States, Germany and Britain are due in Vienna this weekend to join ongoing world power nuclear talks with Iran.
U.S. Secretary of State John Kerry will be joined in the Austrian capital with British Foreign Secretary William Hague and German Foreign Minister Frank-Walter Steinmeier.
Not scheduled to take part in talks in Vienna is Russian Foreign Minister Sergei Lavrov.
Six world powers and Iran are facing a July 20 deadline to complete a deal aimed at stopping Tehran from producing weapons-grade uranium.
The main sticking point is uranium enrichment, a process which can produce nuclear fuel -- Iran's stated aim -- but also material for a nuclear bomb.
Ayatollah Ali Khamenei, Iran's supreme leader, this week suggested Tehran intends to greatly increase its enrichment capacities.
The six powers want a sharp reduction, however, with a senior U.S. official saying recently that Iran's activities in this area should be a "fraction" of what they are now.