White House national security adviser Jake Sullivan said on Wednesday the United States and its allies have made progress in Iran nuclear talks, but issues remain, and it is unclear if they will be resolved.

"We've made progress over the course of the last several weeks. There are still some issues left," Sullivan told reporters aboard Air Force One as President Joe Biden flew to Brussels. He said it is "unclear if this will come to closure or not" but the allies are trying to use diplomacy to put Iran's nuclear program "back in a box."

One of the main issues is the controversial question of removing Iran’s Revolutionary Guard (IRGC) from the US Foreign Terrorist Organization (FTO) list. Iran has been insisting on this demand, while the Biden Administration appears not to have decided yet.

Strong opposition from Republicans and some Democrats in Congress have emerged in recent days, with multiple calls on the White House not to make such a concession.

American regional allies, such as Israel and some Arab states also oppose the move.