"They have been extended twice, we hope that they will not be for a third time," Wang said at a joint news conference with Iranian counterpart Mohammad Javad Zarif.
Wang said the talks between Iran and the P5+1 powers (the United States, Russia, China, Britain and France plus Germany) were a "historic opportunity" to resolve the longstanding dispute over Tehran's nuclear efforts.
"The negotiations have seen positive progress," he said, but "they have become more difficult and complicated" as they near conclusion.
Negotiators are now working toward reaching the political outline of a deal by March 31, with the cut-off point for the technical details of a comprehensive accord by June 30.
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Western powers accuse Iran of seeking to develop a nuclear weapon -- a charge Tehran denies.
Disagreements in the talks centre on the extent of nuclear activities Iran will be allowed to continue and the timetable for the lifting of sanctions imposed on Tehran over its nuclear efforts.