Iran has threatened to restart its uranium enrichment activity after talks with EU negotiators over its nuclear programme ended in deadlock.
Tehran insists its nuclear programme is entirely peaceful
Iranian negotiator Cyrus Nasseri said Tehran may be "forced to resume part of its enrichment programme", after meeting EU counterparts in London.
Both sides are expected to meet again on the sidelines of a nuclear arms control summit in New York on 2 May.
Iran denies US allegations it is aiming to secretly develop nuclear weapons.
Iran has temporarily suspended nuclear enrichment as a confidence-building measure.
France, the UK and Germany - known as the EU Three - have been trying to persuade Iran to abandon enrichment permanently.
But Tehran wants to retain a phased, monitored uranium enrichment programme.
Mr Nasseri said on Saturday that Iran and the EU had made "some progress" in their talks on Friday, but had still not reached an agreement on enrichment.
"We believe there has been some progress on the framework for a long-term agreement... the difference is about the timing," he told AFP.
In November Iran agreed to suspend uranium enrichment for the duration of negotiations, which began in December.
The EU Three want Iran to abandon its uranium enrichment and reprocessing activities, offering a package of political, economic and technological incentives.
A previous round of talks in March ended with no agreement.
The BBC's Pam O'Toole in London says Tehran suspects the Europeans are trying to transform a temporary halt into a de facto permanent suspension by dragging out the talks.
The Europeans have warned they would back US moves to take Tehran to the UN Security Council if Iran breaches agreements or resumes uranium enrichment during the talks.
Iran maintains its nuclear programme is entirely peaceful, but Washington suspects it of secretly trying to build a nuclear weapon.