The move came after the US proposed limited sanctions relief in return for a halt to the most controversial work.
The first meeting in eight months between Iran and the five permanent members of the United Nations Security Council plus Germany took place yesterday in a bid to resolve a dispute that threatens to trigger another war in the Middle East.
Iran has used the last eight months to expand activity the West suspects is aimed at enabling it to build a nuclear bomb, something Israel has suggested it will prevent by force if diplomacy fails.
The negotiations in the Kazakhstan city of Almaty were expected to run through yesterday and today.
However, with the Islamic Republic's political elite preoccupied with worsening infighting before a presidential election in June, few believe the meeting will yield a quick breakthrough.
"It is clear nobody expects to come from Almaty with a fully done deal," a spokesman for the European Union's foreign policy chief Catherine Ashton, who oversees contacts with Iran on behalf of world powers, said shortly after talks started.
A US official said the powers' updated offer to Iran would take into account its recent nuclear advances, but also take "some steps in the sanctions arena".
This would address some of Iran's concerns but not meet its demand that all sanctions be lifted, the official said.
In Almaty, a source close to the Iranian negotiating team said Iran would put up a counterproposal.
The source said: "Depending on what proposal we receive, we will present our own proposal of the same weight. The continuation of talks depends on how this exchange of proposals goes forward."