Iran has held talks with global powers over its controversial nuclear programme, 14 months after a previous round of negotiations were broken off.
Diplomatic sources described Monday's talks, which were held in the Swiss city of Geneva, as "constructive".
The negotiations, which are expected to continue on Tuesday, were spearheaded by Catherine Ashton, the European Union's top diplomat, who is representing the five UN Security Council members - Britain, France, the US, Russia and China - plus Germany.
Officials from each of the countries were present for the talks, including William Burns, the US undersecretary of state.
Bilateral talks were held in the afternoon, with speculation that one might be held between Burns and Saeed Jalili, Iran's chief negotiator.
Tensions rose ahead of the talks when Tehran announced on Sunday that it had mined and enriched its first domestic uranium yellowcake, the raw material needed to produce highly-enriched uranium.
The announcement has been widely interpreted as a signal that Iran will not back down over its nuclear programme, which the West believes aims to build nuclear weapons despite Tehran's insistence that its plans are peaceful.
The talks have also been overshadowed by the death last week in Tehran of a senior Iranian nuclear scientist, who was killed in a bomb attack on his car.
The attack left another scientist wounded and Iranian officials have blamed the blast on the West.
Iran's semi-official Fars news agency said Jalili began Monday's meeting with a reference to the attack, "and a strong condemnation of the terror act".
Diplomatic sources told Al Jazeera that Ashton and the other delegates condemned the attack.
Few observers expect a breakthrough at the Geneva talks.
Michael Adler, from the Woodrow Wilson Centre, termed the discussions "a diminished expectations meeting".
"The Americans just want to get a process going of talking, and of course the Iranian goal - as always - is to get their right to enrich uranium acknowledged so that they don't face this pressure any more from the international community," he told Al Jazeera.
Iran's Jalili, asked by reporters upon his arrival in Geneva whether he was optimistic about the talks, said: "Everything depends on the other party's attitude."
Al Jazeera's Nazanine Moshiri, reporting from Geneva, said Iran and the other countries represented had not even agreed on what they would be talking about.
"The West wants to talk about enrichment, the Iranians are saying that is non-negotiable," she said.
"I think the most interesting thing that will come out of this meeting is possibly a bilateral meeting between William Burns and the Iranians.
"If that happens, that will be extremely important ... not only to discuss the nuclear issue but also [the issue of] the two American tourists who are still in prison in Tehran."
The West has hit Iran with tightened diplomatic sanctions in recent months, and the US has warned of more pressure and isolation if Tehran continues its uranium enrichment activities.
Washington has said that all options, including a military strike, remain on the table if diplomatic pressure cannot be brought to bear on Iran.
But so far Iran has remained defiant in the face of the sanctions, and Sunday's announcement on yellowcake production will do little to ease the Western concerns over the deadlock.