UN Secretary-General Ban Ki-moon
UN Secretary-General Ban Ki-moonReuters

United Nations (UN) Secretary-General Ban Ki-moon on Thursday urged all parties in nuclear talks between Iran and six world powers to show flexibility in order to finally resolve a 12-year dispute over Tehran's atomic program, Reuters reports.

"The Secretary-General calls on all participants to demonstrate the necessary flexibility, wisdom and determination to bring the negotiations to a successful conclusion that meets the concerns and interests of all sides," Ban's press office said in a statement.

"He is convinced that such an accord can contribute to the strengthening of regional and international peace and security at a time when global cooperation is needed perhaps more than ever," the statement said, adding that Ban hoped a deal would "restore confidence in the peaceful nature of Iran's nuclear program."

The statement came as U.S. Secretary of State John Kerry, who travelled to Vienna to join the ongoing talks between Iran and six world powers, said that world powers were not discussing extending the looming November 24 deadline for a nuclear deal with Iran despite being "concerned" about the remaining gaps.

"We are not discussing an extension. We are negotiating to have an agreement. It's that simple," Kerry said in Paris before leaving for the Austrian capital to join the discussions.

He added that, together with British Foreign Secretary Philip Hammond who said Wednesday he was "not optimistic" and suggested an extension was the best they could hope for, he was "concerned about the gaps".

Despite assurances it is confident of reaching a deal by the November 24 deadline, Iran has been toughening its stance in recent weeks. Iran’s chief negotiator, Abbas Araqchi, recently said he sees no prospect for a deal unless the other side abandons its “illogical excessive demands”.

A senior Iranian official followed those comments by declaring that Iran will demand that all Western sanctions be lifted as part of a final deal, rejecting an American proposal of a gradual lifting of sanctions.