Iran said it presented "logical" proposals in talks aimed at achieving a breakthrough in a decade-old standoff over its disputed nuclear programme.
Tehran launched negotiations with the United States, Russia, China, France, Britain and Germany in Geneva this morning.
After years of ideological defiance, Iran appeared keen for a negotiated settlement to win relief from sanctions that have crippled its economy.
Iranian Deputy Foreign Minister Abbas Araqchi said that the global powers had "welcomed" Tehran's proposals and the substantive details would be discussed later today at deputy foreign minister level.
"We think that the proposal we have made has the capacity to make a breakthrough. We had a very serious and good meeting this morning," he told reporters.
"The questions that were asked regarding Iran's plan were completely serious and our answers were as well. Both sides felt that the opposite side was continuing the negotiations with motivation," Mr Araqchi said.
He added: "We are very serious... We are not here to waste time. We are serious for a real target-oriented negotiation."
The West suspects Iran is trying to develop the means to make nuclear arms behind the screen of a declared civilian atomic energy programme.
Tehran denies this but its refusal to limit activity applicable to nuclear bomb production, or to permit unfettered UN inspections, has drawn severe sanctions.
In a possible sign of the Islamic Republic's determination to meaningfully address specifics of the powers' concerns after years of sidestepping them, the talks in Geneva were conducted in English for the first time.
A spokesman for the European Union foreign policy chief, Catherine Ashton, who oversees diplomacy with Iran on behalf of the six nations, said there was a sense of "cautious optimism" ahead of the meeting and that Ms Ashton and Mr Zarif had dinner together yesterday in a "very positive atmosphere".
On the eve of the talks, Washington held out the prospect of quick sanctions relief if Tehran moved swiftly to allay concerns about its nuclear programme, although both countries said any deal would be complex and take time.