The United States has sent its first nuclear deal proposal to Iran since negotiations began in April, a move that comes after a U.N. nuclear watchdog report indicated the Middle Eastern country significantly increased its uranium enrichment over a three-month span.

The U.N. atomic agency reports that Iran hasn’t cooperated with a nuclear probe, raising the prospect of a snapback of sanctions on Tehran.

Iran has dramatically increased the amount of near-weapons grade enriched uranium it possesses, according to a report by the International Atomic Energy Agency circulated Saturday to member states in preparation for the agency’s June board meeting.