Iran has for the first time taken part in high-level discussions on Afghanistan after the US said it had "no problem" with its participation.
An Iranian representative joined the international "contact group" - which brings together the Afghan government, dozens of countries, Nato, the EU and UN - for the talks in Rome.
It comes amid a renewed push to end the bloody nine-year Afghan conflict.
One senior US diplomat said Iran had "a role to play" in tackling the problems.
"We recognise that Iran, with its long, almost completely open border with Afghanistan and with a huge drug problem... has a role to play in the peaceful settlement of this situation in Afghanistan," Richard Holbrooke - the US special representative to Afghanistan and Pakistan - told a news conference.
"So for the United States there is no problem with their presence."
He said discussions would not be affected by the "bilateral issues" of Iran's nuclear programme, which Iran says is for purely civilian purposes but the US insists is a cover for creating atomic weapons.
Iran sent its special envoy for Afghanistan and Pakistan, Mohammed Ali Ghanazadeh, reported Associated Press.
The "contact group" also invited the Organisation of Islamic Conference (OIC), which represents more than 50 states and promotes Muslim solidarity, to attend.
AFP news agency reported Michael Steiner, Germany's special envoy and chairman of the meeting, saying that the talks were aimed at moving towards "a realistic aim, which is sufficient stability for Afghanistan and essential human rights".
On the agenda was how to increase the handing over of responsibility for security to Afghan forces and the possibility of bringing insurgents - including the Taliban - into peace negotiations.
These issues will be further explored at a Nato summit in Lisbon next month. But Mr Holbrooke said there would be no specific announcement of provinces that would be handed over to Afghan forces at the meeting.
"We're not going to lay out specific process," he said. "We're going to talk about the transition process in general."