Speaking after two days of intense talks in Vienna with his Iranian counterpart Mohammad Javad Zarif, Kerry said that despite some "tangible progress", there remain "very real gaps on other key issues."
"I am returning today to Washington to discuss with President Obama and leaders in Congress over the coming days about the prospects for a comprehensive agreement as well as the path forward if we do not achieve one by July 20, including on the question of whether or not more time is warranted, based on the progress we have made and how things are going," he told a news conference.
July 20 is the date when an interim accord struck between Iran and the five permanent members of the UN Security Council plus Germany expires, meaning that a final accord has to be nailed down by then.
Extending the deadline has always been a possibility in order to keep the parties talking, but Washington in particular has stressed it will not agree to such a move without key concessions from Iran first.
Also Read
Iran denies seeking the atomic bomb and wants the lifting of crippling UN and Western sanctions.
The six powers want Iran to dramatically reduce in scope its nuclear programme for a lengthy period of time and agree to more intrusive UN inspections.
This would greatly expand the time needed for the Islamic republic to develop a nuclear weapon, should it choose to do so, while giving the world ample warning of any such "breakout" push.