Iran will reach a nuclear deal with world powers if the other side does not make "excessive demands," Iranian Foreign Minister Mohammad Javad Zarif said in Vienna on June 27 for the final stage of talks.
Zarif held talks with U.S. Secretary of State John Kerry ahead of a self-imposed deadline of June 30, which officials expect to slip by at least two or three days.
Zarif said negotiators "need to work really hard in order to be able to make progress and move forward."
He said that Washington needed to stand by its commitments.
"We will continue to insist on the fact that the other side needs to implement and abide by its commitment, particularly on the removal of sanctions -- all of the sanctions," he said.
"Economic and financial sanctions need to be removed immediately. And sanctions, Security Council sanctions, must also be terminated immediately."
Kerry told reporters that although he remained "hopeful" ahead of the deadline but added "we've got a lot of hard work to do."
Senior Iranian negotiator Abbas Araqchi said the negotiations have been "slow and difficult."
He suggested parts of a framework deal reached in April in Lausanne could no longer be applied because other countries had changed their positions.
"In Lausanne we found solutions to many things, but some issues remained unresolved," he told an Arabic-language Iranian television channel, Al-Alam.
"And now some of the solutions found in Lausanne no longer work, because after Lausanne certain countries within the P5+1 made declarations...and we see a change in their position which complicates the task."
Iran and six world powers are seeking an agreement that would curb Tehran's controversial nuclear program in exchange for relief from economic sanctions.
Significant gaps remain over the sequencing of sanctions relief and the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement.
European Union foreign policy chief Federica Mogherini arrived in Vienna on June 28 to attend the talks.
Foreign ministers from Britain, China, France, Germany, and Russia are expected to "check in" for the talks as well.
They must settle disputes over the speed and scope of lifting sanctions, how Iran will reduce its stockpiles of low-enriched uranium, and access for United Nations inspectors to Iranian nuclear scientists and military sites, among other issues.