Rafael Grossi, head of the International Atomic Energy Agency, the U.N. nuclear watchdog, says the window of opportunity to diplomatically engage with Iran is beginning to "shrink."

Rafael Mariano Grossi of the IAEA was visiting Tehran in an effort to restore his inspectors' access to Iran's program and answer still-outstanding questions over it, as he has on previous trips with limited success since Trump unilaterally withdrew the U.S. from the Islamic Republic's nuclear deal with world powers.