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.

"The possibility of reaching a deal is very high, and a deal should be agreed," Wang told reporters.

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 tack 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.