The head of the United Nations’ nuclear watchdog prepared to meet Saturday with Iranian officials as talks in Vienna over Tehran’s tattered atomic deal with world powers appear to be reaching their end.

Axar.az reports that Rafael Mariano Grossi described his weekend visit to Tehran as a means “to address outstanding questions” as negotiators back in Europe appear to be reaching a deadline to see if the 2015 accord can be revived.

“This is a critical time but a positive outcome for everyone is possible,” Grossi wrote on Twitter ahead of his flight Friday.

The nuclear deal saw Iran agree to drastically limit its enrichment of uranium in exchange for the lifting of crushing economic sanctions. But a 2018 decision by then-President Donald Trump to unilaterally withdraw America from the agreement sparked years of tensions and attacks across the wider Mideast.

Today, Tehran enriches uranium up to 60% purity its highest level ever and a short technical step from weapons-grade levels of 90% and far greater than the nuclear deal’s 3.67% cap. Its stockpile of enriched uranium also continues to grow, worrying nuclear nonproliferation experts that Iran could be closer to the threshold of having enough material for an atomic weapon if it chose to pursue one.