Foreign ministers from major world powers resumed talks in Vienna, one day before a self-imposed deadline to reach a nuclear deal with Iran.
Top diplomats from Iran and the United States, Russia, China, Britain, France, and Germany gathered at the Palais Coburg Hotel to try to close the last gaps on the deal.
The accord would limit Iran's civilian nuclear activities in exchange for relief from international sanctions.
Iranian Foreign Minister Mohammad Javad Zarif told reporters on July 6 that "some differences remained" between the two sides.
U.S. Secretary of State John Kerry said on July 5 that "genuine progress" had been made in the talks but that "several of the most difficult issues" remain and it could go "either way."
Chinese Foreign Minister Wang Yi said on July 6 that a "comprehensive agreement is within reach."
Kerry said on July 5 that "it's now time to see whether or not we are able to close an agreement."
Kerry made the comments after a meeting with Zarif.
"While I completely agree with Foreign Minister Zarif that we have never been closer, at this point this negotiation could go either way," he said.
He said that the United States is prepared to walk away from the talks if there is absolute intransigence.
Kerry said no one wants that scenario, but Washington won't "shave at margins" to clinch an insufficient package.
Speaking several hours later, Zarif said that some differences still remained in the nuclear talks with major powers.
"Still nothing is clear...some differences remain and we are trying and working hard," Zarif told reporters.
Iran and world powers have reportedly made progress on future sanctions relief for Tehran in the marathon nuclear talks but remain divided on issues such as accounting for Iran's past nuclear activities.
Earlier, diplomats said they had tentative agreement on a mechanism for suspending U.S. and European Union sanctions on Iran.
But the six powers had yet to agree on a United Nations Security Council resolution that would lift UN sanctions and establish a means of reimposing them in case Iran does not comply with a future agreement.
"We don't have Iran on board yet," a diplomat said, though technical experts have drawn up a draft annex to the emerging nuclear agreement that would establish the pace and timing of sanctions relief.
Abbas Araqchi , Iran's lead negotiator, said there were still "four or five" outstanding questions regarding sanctions, including synchronizing their lifting with actions by Tehran to curb nuclear activities.
"There are complications related to the simultaneous lifting of sanctions with the execution of Iran's technical obligations," he said.
The negotiations call for Iran to start cutting back on its nuclear program after an agreement is reached, possibly as early as next week.
But that would take up to four months, Araqchi said, and he insisted that Iran is not willing to wait that long for the crippling sanctions to be lifted.
"That would be a problem for us," he said.
Araqchi issued a warning that the talks could still collapse.
The sanctions relief offered by the United States, Britain, China, France, Germany, and Russia would come in exchange for Iran curtailing its nuclear program for at least a decade.
The negotiators missed a June 30 deadline for a final agreement, but have given themselves until July 7.
Western and Iranian officials said there were signs of a compromise emerging on another major sticking point: access to Iranian sites to monitor compliance with a future agreement.
Another potential emerging compromise relates to Iran's stockpile of low-enriched uranium.
Western and Iranian diplomats said Tehran was considering shipping most of the stockpile out of the country, something Tehran had previously ruled out.
Russian diplomats have said the complex accord, which will stretch to at least 20 pages with a slew of technical annexes, is "90 percent" written.
But multiple sticking points remain.
One is a stalled UN investigation into the possible military dimensions of past Iranian nuclear research suspected of being linked to weapons development.
Another is Iran's demand to continue research and development work on advanced centrifuges, machines that purify uranium for use as fuel in power plants or weapons.
UN International Atomic Energy Agency Director-General Yukiya Amano said he could issue a report on its investigation into past Iranian research suspected of being linked to nuclear weapons development by the end of the year -- but only if Tehran cooperated.
Western diplomats said they were not demanding a public confession that Iran had conducted research into building a nuclear warhead.
But they said the UN agency had to be satisfied it knew the full scope of past Iranian activity to establish a credible basis for future monitoring.
With reporting by Reuters, AFP, dpa, and AP