Although the talks are not intended as a fundraiser, Clinton said she planned to formally pledge $40m to fund Afghanistan's elections.
Hamid Karzai, the Afghan president, speaking at the opening of the conference on Tuesday, welcomed commitments to prevent civilian casualties and urged his country's allies to invest in the development of national security forces.
"Building up the Afghan security capacity will be the surest, most sustainable and least costly way to overcome the threat of terrorism, provide security and law and order to the people of Afghanistan.
"The Afghan people also expect that any military scale-up will be used effectively to protect civilian lives and to stem the infiltration of terrorists from across Afghanistan's borders," he said.
James Bays, Al Jazeera's correspondent at The Hague, said he understood Karzai had met with Richard Holbrooke, the US special envoy.
"We don't know exactly what they discussed. But members of the Afghan delegation told Al Jazeera that was a stormy meeting and that the president was extremely angry at the end of it," he said.
He added that ordinary Afghans want the conference to produce something that is going to change their lives.
Clinton is expected to ask conference delegates for their countries' support for Washington's new military strategy in Afghanistan, which aims at stabilising the country, unveiled last week by Barack Obama, the US president.
Obama said he would send an extra 17,000 soldiers and 4,000 advisers to train Afghan forces, plus hundreds more civilians to work on development issues.
But Iran has voiced objection to the presence of foreign troops in Afghanistan.
Mohammad Mehdi Akhoundzade, the Iranian deputy foreign minister, was quoted by Iran's official Irna news agency as saying: "The presence of foreign troops cannot bring peace and stability for Afghanistan."
He instead called for a regional solution.
The Red Cross has also warned that the planned increase in military operations is likely to mean more civilian casualties.
The aid group urged the conference to "consider the plight of civilians as a matter of urgency".
Meanwhile, rights group Amnesty International called for steps to be taken to improve conditions for Afghans.
"The Afghan people deserve and demand performance, not promises, from their government and its international supporters, chief among them the United States," Sam Zarifi, director of Amnesty International's Asia-Pacific programme, said.