Negotiators from Iran and several European countries have resumed talks in Geneva on resolving the international standoff over Tehran's nuclear program.
Iran insists it has the right to enrich uranium, which can be used in energy production and nuclear weapons. Britain, France and Germany say Iran must permanently abandon such activities.
Iranian officials have said they will never agree to such a halt.
The latest round of confidential talks is scheduled to continue through Thursday.
Last week, diplomats at an International Atomic Energy Agency meeting in Vienna said Iran is building a system of underground tunnels to conceal and protect parts of its nuclear program.
The United States maintains Iran is trying to covertly develop nuclear weapons, but Tehran says its program is meant only to meet the country's energy needs.
Some information for this report provided by AFP and Reuters.