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.

At an international conference in Tehran, top Iranian officials said they were eager to make a deal with Trump but that they wouldn’t buckle under pressure.