Geneva, Switzerland (CNN) -- Iranian officials sat down Monday with the United States and other countries trying to put the brakes on Tehran's nuclear program, a day after Iran announced it is self-sufficient in the nuclear fuel cycle.
There was "an exchange of views and concerns," between the Iranians and envoys from the five permanent members of the U.N. Security Council, plus Germany, a diplomatic source familiar with the talks told CNN.
The Iranian nuclear program was the main issue on the table, the source said. Iran has previously said it did not want the talks to focus on that.
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 diplomatic source and a Western official later both told CNN that the talks had ended for the day and would resume Tuesday.
The United States and other countries fear that Iran wants to develop nuclear weapons, an allegation Tehran has always denied.
The meeting in Switzerland is between Iran on the one hand and the so-called P5 plus 1 group -- the United States, China, France, Germany, Russia and United Kingdom -- on the other.
But bilateral talks also are going on, the diplomatic source said, refusing to say who was talking to whom. The source asked not to be identified because of the sensitivity of the negotiations.
The talks are going on behind closed doors and no public statements have been made so far.
They'll continue Tuesday, the source said. And the P5 plus 1 nations are prepared to offer more talks if the Iranians agree to focus seriously on the nuclear issue.
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 Majid Shahriari and injured 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."
Salehi also said that Iran is now producing yellowcake, meaning its nuclear program is self-sufficient since it now produces everything needed to make nuclear fuel. He said the yellowcake came from the Gachin mine near Gander Abbas, on Iran's southern coast near the Strait of Hormuz.
It's not clear that Iran actually has the technology to complete the entire nuclear fuel cycle, despite Sunday's announcement.
But U.S. officials weren't surprised by the announcement, with Hammer saying, "Iran has been trying to develop an indigenous program for years given that the import of yellowcake is banned by United Nations Security Council resolutions."
He said the move raises further concerns about the Middle Eastern country's intentions, "given that Iran's own supply of uranium is not enough for a peaceful nuclear energy program."
CNN Senior International Correspondent Matthew Chance contributed to this report.