"The key issues are quite clear and the preferred approach of a solution is emerging," Wang told reporters as he left the luxury lakeside hotel in Lausanne, Switzerland, where the talks are being held.
"The differences between the positions of the parties are narrowing and I'm cautiously optimistic on the prospects," he added.
Asked whether a deal could be reached by tomorrow's midnight deadline, Wang said he hoped there would be "this outcome" but said that "for this to happen everybody should contribute."
"We hope a comprehensive agreement can be reached as soon as possible so as to help safeguard the international nuclear non-proliferation regime and help the people of Iran escape the burden of sanctions," Wang added.