US Secretary of State John Kerry attends a press conference in Vienna, Austria, Nov 24, 2014. [Photo/Xinhua]

VIENNA - Sides will meet again in December to discuss the tough Iranian nuclear issue, and the expert-level meeting will start soon, aiming at an political deal within four months, US Secretary of State John Kerry told reporters on Monday.

Kerry said that expert-level talks will start soon. "In fact we will have meetings in December," he noted.

Speaking at a press conference, Kerry said sides had achieved "real and substantial progress" over the Iranian nuclear issue in the past negotiations, which lays foundations for further talks in the next months.

"These last days in Vienna, we have made real and substantial progress, and we have seen new ideas surface," Kerry said.

After one week of difficult talks, parties involved decided to extend Iran nuclear talks by seven months, aiming to reach a political agreement within the next 4 months, Kerry said.

Kerry stressed that even the talks are not finalized "the world is safer than it was just one year ago".

"We are closer to a deal," he said, adding that comprehensive agreement is "possible" and "desirable".

An interim deal was agreed in Geneva last November, under which Iran suspends some sensitive nuclear activities in exchange of limited sanction relief to buy time for the diplomatic effort to resolve issue.

Iran currently has installed 19,000 centrifuges, of which 10, 190 are operating. Most of the centrifuges Iran owns are IR-1, an old model centrifuge, and Iran has also installed around 1,000 IR- 2M centrifuges, which are assessed to be three to five times more efficient than IR-1 centrifuges.

Israel openly questions the nuclear talks, fearing that a "bad nuclear deal" could undermine its security, leaving open the possibility of military action against Iranian nuclear facilities.