"The negotiations continued until 4:30 am (0330 GMT) in Geneva and the two sides have made good progress on different issues, but we cannot prejudge the results before the end of the negotiations," lead negotiator Abbas Araqchi said.
He added that there would "probably be another round of negotiations before noon and before announcing the results."
Experts from Iran and the so-called P5+1 -- the United States, Britain, France, Russia and China plus Germany -- have been holding technical talks on implementing an agreement reached November 24 on Iran's controversial nuclear programme.
Western nations and Israel have long suspected Iran is covertly pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran, which insists its uranium enrichment is for purely peaceful purposes.