UN nuclear inspectors and Iran have agreed to meet again on January 16, but there was no breakthrough reported at talks on December 13 in Tehran.
The Iranians said some progress had been made.
Herman Nackaerts, the deputy director of the International Atomic Energy Agency (IAEA), said the talks could lead to a deal next month on how to conduct an investigation.
The IAEA is investigating whether Iran is using its nuclear program to develop nuclear weapons.
The IAEA is seeking access to the Parchin military complex, where the agency believes Iran has conducted explosives tests with possible nuclear applications.
There was no indication that Iran was finally willing to grant the IAEA access to Parchin after the December 13 talks.
Tehran says the IAEA inspected Parchin in 2005 and found no evidence of secret nuclear work.
Satellite images taken this year indicate buildings have been demolished and soil removed at Parchin, erasing possible traces of illicit activity.
Meanwhile, the United States has imposed more sanctions on Iran over its nuclear program.
The latest sanctions announced by the Treasury Department target seven companies and five individuals accused of supporting Iran's nuclear program.
The sanctions bar those companies and individuals from doing business with U.S. firms or citizens and freeze any assets they have in the United States.
The Treasury Department said the newly announced sanctions were aimed at targeting the international procurement operations of the Atomic Energy Organization of Iran, the Iran Centrifuge Technology Company, and Iran's uranium-enrichment program.
Based on reporting by AFP, Reuters, and dpa