Iran’s supreme leader has criticized an initial proposal from the United States in negotiations over Tehran’s rapidly advancing nuclear program, though he stopped short of entirely rejecting the idea of agreement with Washington.

Iran’s Supreme Leader Ayatollah Ali Khamenei on Wednesday slammed a preliminary proposal from the United States in talks about Tehran’s nuclear program. However, he did not completely dismiss the possibility of reaching an agreement with Washington.

An outline by the Trump administration would allow Iran to continue enriching uranium at low levels while a broader arrangement is worked out that would block the country’s path to a nuclear weapon.