Geneva, Switzerland (CNN) -- The nuclear talks in Switzerland between Iran and the nations known as the P5 plus 1 ended Tuesday, officials said.
The group agreed to hold another round of talks in Istanbul, Turkey, early next year, said Saeed Jalili, Iran's top nuclear official.
The Iranian nuclear program was the main issue on the table during the talks, a diplomatic source familiar with the talks said. Iran has previously said it did not want the meeting to focus on that topic.
Iran also raised some of its concerns, the source said, including attacks on two Iranian scientists in Tehran last week that left one dead and one injured.
The United States and other countries fear that Iran wants to develop nuclear weapons, an allegation Tehran has always denied.
The meetings in Switzerland were between Iran and the so-called P5 plus 1 group -- the five permanent members of the U.N. Security council -- the United States, China, France, Russia and the United Kingdom -- plus Germany.
The group has been meeting intermittently, with the last round of talks coming over a year ago.
Mike Hammer, a spokesman for the U.S. National Security Council, said Sunday the talks would try "to underscore the concern of the entire international community in Iran's actions and intentions."
Iran already faces stiff sanctions from the international community, because it has continued to enrich uranium.
With the political barbs flying back and forth regularly, tensions were raised even further last Monday when bombers targeted two Iranian nuclear scientists.
Iran blamed Israel, the United States and the United Kingdom's spy agencies for the attacks, which killed Professor Majid Shahriari and injured Professor Fereydoun Abbasi.
But Ali Akbar Salehi, the head of Iran's Atomic Energy Organization, told state-run Press TV on Sunday that the "assassination of Iranian scientists will not hamper our progress."
