While Iran had said its nuclear program would not be off-limits at the Geneva talks, one official, whose name was not reported, told The Washington Post "70 (percent) to 80 percent" of more than 3 hours of morning talks dealt with the country's nuclear ambitions.
The afternoon was taken up by bilateral meetings before the groups reconvened for another 90-minute session dealing with an Iranian presentation.
A U.S. official told the Post that while the United States did not have a one-on-one meeting with Iran, others, including Russia and China, "did and delivered the unified message" that Iran needed to address international concerns about its nuclear program. Britain, France and Germany also are party to the talks.
The meetings, which initially had been scheduled for a single day, were described as "constructive" and "positive," and are to continue Tuesday morning, a Western official said. That could indicate some progress was being made during the closed-door talks.
The Post said officials and the state-run Iranian news agency Mehr reported Saeed Jalili, Iran's top negotiator, spoke about the recent slaying of one of his country's best nuclear scientists and stated Iran's mistrust of the countries represented at the meeting.
Catherine Ashton, the European Union foreign policy chief, said the six nations condemned the assassination.
Iran had said Sunday it will begin using domestically produced uranium concentrate at its enrichment facilities, which it contends are only for peaceful purposes.
In a speech Sunday in Tehran, Iranian President Mahmoud Ahmadinejad said the era of using atomic bombs is over. He criticized Western powers for having nuclear weapons as they try to punish Iran for the same thing, Ahlul Bayt News Agency reported.
"The United States launched wars in Iraq and Afghanistan, but did not use atomic bombs because the era of using nuclear bombs has come to an end," Ahmadinejad was quoted as saying by Islamic Republic of Iran Broadcasting.
"You have a thousand A-bombs," he said. "How is it that you are worried that Iran might be able to develop atomic bombs in three years' time?"
Iran has insisted it isn't interested in discussing its enrichment program, which resulted in four rounds of U.N. Security Council sanctions.