Mrs Clinton said Iran's presence at The Hague meeting was 'a promising sign'
The US envoy to Afghanistan has held "a cordial exchange" with Iran's deputy foreign minister, US Secretary of State Hillary Clinton has said.
The meeting between Richard Holbrooke and Mohammad Mehdi Akhoondzadeh took place on the sidelines of a meeting in The Hague to discuss Afghanistan.
The US has welcomed Iran's presence at the meeting.
Analysts say the easing of tensions between the US and Tehran is due to a new tone set by the Obama White House.
Earlier, Iran gave a guarded welcome to US plans to increase regional co-operation over Afghanistan.
"In the course of the conference today, our special representative for Afghanistan, Richard Holbrooke, had a brief and cordial exchange with the head of the Iranian delegation," Mrs Clinton told a news conference.
She said the meeting had been unplanned, but Mr Holbrooke and Mr Akhoondzadeh had agreed to "stay in touch".
She added that Iran's presence at Tuesday's meeting was "a promising sign that there will be future co-operation".
Mrs Clinton was speaking at the end of the one-day meeting of delegates from 70 countries and other organisations interested in rebuilding Afghanistan.
The meeting was called by the UN amid widespread concern that not enough progress has been made since the US-led invasion in 2001.
Mohammad Mehdi Akhoondzadeh criticised US plans for a troops 'surge'
During the conference, Afghan President Hamid Karzai said much of the aid given to his country had been wasted.
He said the failure to deliver aid effectively was a cause of widespread grievance among Afghans.
In response, Mrs Clinton said aid should be more effectively spent, but added that the Afghan state needed to be more accountable and had to tackle corruption.
She said the cost of remaining in Afghanistan in the long term might be high, but it would be worth it.
"Let us be guided by an ancient afghan proverb - patience can be bitter, but its fruit is sweet," she said.
BBC international development correspondent David Loyn says US President Barack Obama's new Afghan strategy was given a broad welcome at the conference.
Mr Akhoondzadeh said Iran was fully prepared to participate in "projects aimed at combating drug trafficking and the plans in line with developing and reconstructing Afghanistan".
However, he was critical of US plans to send more troops to the region, saying the money would be better spent on building Afghanistan's own forces.
"The presence of foreign forces has not improved things in the country, and it seems than an increase in the number of foreign forces will prove ineffective, too," he said.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.