The United States and Iran concluded the first of two days of bilateral talks in Geneva on Sept. 4, as some analysts said prospects for reaching a nuclear deal are growing less likely amid continued impasse on the issue of the size of Iran's enrichment capacity.

“Primarily because the sides are at an impasse over the size of Iran's enrichment capacity, Eurasia Group now believes that a final deal is unlikely,” the Eurasia Group’s Cliff Kupchan wrote in an analysis for the risk consultancy Sept 4. “However, this remains a very close call and last minute agreement is possible.”

US negotiators believe they still have a shot at getting a deal, but say the toughest issue remains closing the gap with Iran on the issue of the size of its enrichment capacity in a final deal. Western diplomats are also frustrated at Iran’s slow progress in cooperating with the International Atomic Energy Agency (IAEA) in answering questions about suspected past possible military dimensions (PMD) of its nuclear program.

Deputy Secretary of State Bill Burns, Undersecretary of State Wendy Sherman and senior adviser Jake Sullivan, the former national security adviser to Vice President Joe Biden, led the 11-member US delegation to the meeting with the Iranians in Geneva Thursday, the State Department said. The Iran delegation was led by their deputy foreign ministers and nuclear negotiators Abbas Araghchi and Majid Ravanchi.