VIENNA, Nov. 25  -- Iran and six world powers on Monday agreed to extend negotiations on the Iranian nuclear issue by seven months after days of intensive talks have failed to produce a comprehensive deal.

EU envoy Catherine Ashton and Iranian Foreign Minister Mohammad-Javad Zarif on Monday night announced the decision to postpone the deadline till June 30.

More work is required to assess and finalize ideas developed during meetings and talks in the past months, they said in a joint statement.

"We intend to build on the current momentum in order to complete these negotiations within the shortest possible time, up to four months," said Ashton. If necessary, she said, the negotiators will use the remaining three months to finalize any possible remaining technical and drafting work.

Iran and the six countries -- Britain, China, France, Germany, Russia and the United States -- will continue to implement all their commitments described in a Joint Plan of Action, and the International Atomic Energy Agency will be asked to continue monitoring the voluntary measures under the deal, said Ashton, former security and foreign policy chief of the European Union.

Negotiators from Iran and the six world powers, who have been in talks in the Austrian capital since Tuesday, have failed to reach a long-term solution to Iran's nuclear program by a deadline set for Monday.

"Everyone has worked hard, trying to overcome the differences," Zarif told reporters following Ashton's statement, saying the negotiating parties want to keep the current positive atmosphere to further promote nuclear talks.

Noting that only a negotiated solution can resolve the nuclear issue, he said "hard decisions" have to be made, and flexibility needs to be shown by all sides.

Iran's nuclear program has long been suspected by Western powers, which demanded Iran significantly scale back uranium enrichment to address their concerns, while Iran argues that its nuclear program is for peaceful purposes only.

Iran and the world powers agreed on the Joint Plan of Action in Geneva last November to halt Tehran's nuclear program in exchange for limited easing of sanctions.

After failing to narrow down gaps on core issues during months of negotiations for a comprehensive deal, Iran and the powers agreed in July to extend the nuclear talks for another four months till Monday.

U.S. Secretary of State John Kerry said Monday that Iran was indeed living up to its commitments by freezing its nuclear program, adding that all the progress made in the past year lays a foundation for a comprehensive deal.

He was optimistic about the outcome of the talks, saying it is "absolutely possible" to reach a comprehensive agreement in the end. The sides need more time to close gaps in some areas and to work through all the possible solutions to the issue, he said.

According to the statement by Ashton and Zarif, the parties are expected to meet again in December.

Chinese Foreign Minister Wang Yi said Monday that progress made in the Vienna talks has built a strong foundation for reaching a final deal, and that China is optimistic about the prospect of the negotiations.

Noting that the issue is very complicated and involves lots of technical details, Wang urged all parties concerned to keep their determination to resolve it through joint efforts, adhere to the existing consensus, meet each other halfway and show flexibility so as to reach an agreement that is acceptable to all parties.