Iran's foreign minister was more cautious as the talks opened, saying they were meant merely to exchange ideas.
The two sides hope to reach an agreement by July that eases international concerns about Iran's nuclear-weapons making capacity by trimming and strictly monitoring its atomic programmes.
Tehran denies any interest in nuclear weapons but is looking for a deal that will give it full relief from sanctions imposed progressively as it expanded its nuclear activities over the past decade.
But Ashton's spokesman, Michael Mann, described the dispute as a possible "misunderstanding" that would not affect the meeting.
Also Read
Zarif leads his country's delegation, while Ashton negotiates at least formally on behalf of the United States, Russia, China, Britain, France and Germany.
Iranian media reported yesterday that Zarif canceled his usual pre-talks dinner with Ashton to protest her meeting with opposition figures.
Going into the morning round of talks, Zarif was cited by Iran's official IRNA news agency as saying they "will be merely an exchange of ideas." But Mann said they focused on the "nitty-gritty" of preparatory work done by experts on both sides.