Six world powers made Iran an offer if it stopped processing medium-enriched uranium, which can be used to make nuclear weapons, EU officials said.
But Iran says uranium enrichment is its non-negotiable right.
A BBC correspondent at the talks says no-one is expecting any immediate breakthrough.
Negotiators for the five permanent members of the UN Security Council plus Germany presented Iran with a package combining new and old proposals, an unnamed Western diplomat quoted by the Associated Press news agency said.
It included an offer of medical isotopes and co-operation on nuclear safety.
In exchange, Tehran would stop its 20% uranium enrichment programme as a first step, the diplomat said. Read More