The "snapback" re-imposition of U.N. sanctions on Iran lifted under a 2015 nuclear deal sends Western powers back to the drawing board on how to contain and monitor Iran's nuclear program, diplomats and analysts say.

European powers France, Britain, and Germany — known as the E3 — had hoped the threat of snapback would make Iran yield to demands such as quickly letting U.N. nuclear inspectors return to nuclear facilities bombed by Israel and the U.S. in June, and resuming talks with the United States on its atomic activities.

But despite a last-minute rush of diplomatic activity at the U.N. General Assembly in New York, the E3 said Iran had done too little for them to stop snapback going into effect on Sunday, a month after they set the 30-day process in motion.