A U.S. envoy says Washington is disappointed with Iran over the degree of cooperation it has shown with the U.N. nuclear agency's attempts to probe whether Tehran ever worked on atomic arms.

A report this month from the U.N.'s International Atomic Energy Agency said the investigation has made no headway for months despite pledges by Iranian leaders to cooperate.

The IAEA has tried to follow up on the allegations for more than a decade, with little success. Iran denies it has any interest in -- or is working on -- such weapons.

U.S. envoy Laura Kenney said Monday Washington is disappointed with Iran's "failure thus far, to constructively engage on this issue."

She spoke a day before the start of talks focused on reducing the scope of Tehran's nuclear programs.