"As we lay the groundwork for these talks, we will keep up the pressure as part of our dual-track approach. All of our sanctions will remain in place and continue to move forward during this period," she told reporters in Washington hours after talks between Iran and world powers concluded in Baghdad.
Iran and world powers agreed to meet again in Moscow on June 18-19 for more negotiations to try to end the long dispute over Tehran's nuclear program but there appeared to be scant progress to resolve the main sticking points between the two sides.
At the heart of the dispute is Iran's insistence that it has the right to enrich uranium and that economic sanctions should be lifted before it stops activities that could lead to its achieving the capability to make nuclear weapons.
Western powers insist Tehran must first shut down enrichment activities before sanctions can be eased.
Clinton was guarded in assessing the Baghdad talks, but seemed heartened that the major powers - Britain, China, France, Germany, Russia and the United States - were able to hold two rounds of serious discussions with Iran after more than a year.
"There are clearly gaps on what each side sees as possible and, you know, we think that the choice is now Iran's to work to close the gaps," Clinton said. "It's very clear that there is a lot of work still to do.
"And yet, at the same time, I have to say that this is the second of two serious meetings after a gap of at least 15 months where there was no contact and no discussion about any of these matters," she added.