Iran and the major powers reached a framework agreement in Lausanne on April 2, which specified many of the key elements of the final deal, but differences exist between Iran and the Western states in its interpretation.

Wang said that the text of the comprehensive deal has basically taken shape, saying there is a general understanding among the convening parties to stick to the Lausanne framework agreement.

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

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

Progress has been reported in the monitoring and verification issue of Iran's nuclear plan, after International Atomic Energy Agency (IAEA) head Yukiya Amano met with Iranian President Hassan Rouhani in Tehran on Thursday.

Iran will resolve remaining technical nuclear issues in close cooperation with the IAEA in a specific timetable, Rouhani said on the same day.

Rouhani and Amano discussed how to speed up resolution of the concerns about Tehran's past nuclear activities, Iranian media reported.

The IAEA chief also met with Ali Shamkhani, secretary of Iran's Supreme National Security Council.

The real deadline of the deal could be expanded further to July 7 rather than the announced June 30.

If a deal is presented after July 7, the US Congress would have the power to review it for 60 days rather than 30 days, increasing risks for the hard won deal to unravel.