After meeting with his Iranian counterpart Mohammad Javad Zarif on Sunday, two days ahead of the July 7 deadline, Kerry told reporters that it is now time to see whether or not he and his foreign minister colleagues returning to Vienna are able to close an agreement.



However, despite genuine progress made in the past few days, Kerry said consensus are still not reached on several of the most difficult issues.



"At this point, this negotiation could go either way. If hard choices get made in the next couple of days, and made quickly we could get an agreement this week. But if they are not made, we will not," Kerry said.



"We want a good agreement, only a good agreement, and we're not gonna shave anywhere at the margins in order just to get an agreement. Right now we're aiming to try to finish this in the timeframe that we've set out. That's our goal, and we're gonna put every bit of pressure possible on it to try to do so," he said.



Meanwhile, European Union foreign policy chief Federica Mogherini said the foreign ministers will meet again on Monday for the negotiation.



"My message today is that, if the deal can be closed, it's now. It's not time the issue, the issue is political decisions that need to be taken now," said Mogherini.



China remains optimistic. Chinese Foreign Minister Wang Yi said on Thursday that a comprehensive Iranian nuclear deal is very likely to be reached after intensive meetings between Iran and six world powers.



Despite the optimism, gaps remain in some areas of the talks, especially on the pace and timing of sanction relief.



Earlier, reports said that Iran may change track on the Lausanne framework, raising concern of a possible setback in the talks.



Iran and five United Nations Security Council permanent members plus Germany (P5+1) missed the previously set self-imposed June 30 deadline for a comprehensive deal. Both sides agreed to extend the deadline to July 7.



All parties have been negotiating over the past 16 months to reach a long-term deal over Tehran's disputed atomic plan.



In a successful deal, Iran would suspend some sensitive nuclear activities, and in return, Western states would partially relieve some sanctions imposed on Tehran.