President Barack Obama said the United States and Iran have narrowed their differences in nuclear weapons negotiations.
However, Mr Obama repeated that he was willing to walk away from negotiations if Tehran would not meet Washington's demands.
His comments came as the Republicans renewed their warning that any deal will face a tough congressional review.
The talks centre on restricting Iran's ability to produce weapons-grade nuclear materials, while allowing it to develop nuclear energy.
Iran says its nuclear program is for peaceful purposes, but Washington and others fear it is seeking the capacity to build a bomb.