She told delegates it was "in the interests of all of the people we represent".
And she linked progress in Afghanistan to progress fighting extremism in Pakistan, echoing the new unified approach to the region recently announced by US President Barack Obama.
"In Afghanistan and Pakistan we face a common threat, common enemy and common task," she said - but she also pledged to offer a "an honourable form of reconciliation" to members of al-Qaeda and the Taleban who abandoned violence.
Opening the conference, Afghan President Hamid Karzai welcomed regional interest in his country's development.
Relations between Mr Karzai and his US backers have seen some friction of late, after some US officials questioned the effectiveness and honesty of his government.
Mr Karzai hailed the "recognition that without the true co-operation of Afghanistan's neighbours, the victory over terrorism cannot be assured".
He highlighted in particular "the close partnership we have developed [with] the democratically-elected government of Pakistan", saying it had become "a valuable asset to the regional approach to fighting terrorism".
The presence of Iran's deputy foreign minister, Mohammad Mehdi Akhoondzadeh, has been applauded by the US.
The US is reinforcing its troop numbers in Afghanistan
"Iran is fully prepared to participate in the projects aimed at combating drug trafficking and the plans in line with developing and reconstructing Afghanistan," he said, according to AFP news agency.
But earlier, he said Afghans held the key to the future of their nation, not the international troops fighting the Taleban.
"The presence of foreign troops can't bring the peace, security and stability to the country," he told Iranian state media.
However, the BBC's international development correspondent, David Loyn, says that behind Iran's routine criticism lays a far more nuanced approach.
Mr Akhoondzadeh said Iran had increased the capacity of the docks at Chabahar - its nearest port to Afghanistan - to allow non-military supplies to travel into Afghanistan via Iranian territory.
Our correspondent says the cautious overture by Iran comes in response to the new mood set by the Obama White House.
The US says it will contribute $40m (£28m) toward holding the Afghan elections this summer.
Last week, Mr Obama announced a fundamental rethink of US strategy in Afghanistan and Pakistan to combat an "increasingly perilous" situation.
He said growing radical forces in the area posed the greatest threat to the American people and the world.
Promising an extra 4,000 US personnel to train and bolster the Afghan army and police, he also vowed support for civilian development.
Special US envoy Richard Holbrooke, also attending the conference, earlier predicted it would be "the launch point for the international recommitment to the effort in Afghanistan and western Pakistan".
Tuesday's conference is officially billed as a "Comprehensive Strategy in a Regional Context".
Of 73 countries invited, only one, Uzbekistan, declined to attend.
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