She made the comments at a global conference on Monday in Germany on Afghanistan's future. Some 100 nations and international organizations are participating.
The conference is focused on the transfer of security responsibilities, international aid and a possible political settlement with the Taliban.
"Together we have spent blood and treasure in fighting terrorism," Afghan President Hamid Karzai said. "Your continued solidarity, your commitment and support will be crucial so that we can consolidate our gains and continue to address the challenges that remain. We will need your steadfast support for at least another decade," Karzai added, echoing an assessment by the World Bank.
The conference was overshadowed by a public display of bad blood between the U.S. and Pakistan, the two nations with the greatest stake and say in making Afghanistan safe and solvent.
Pakistan boycotted the conference, and Islamabad decided not to attend the conference to protest an apparently errant U.S. air strike last month that killed 24 Pakistani soldiers along the rough border with Afghanistan.
"It was unfortunate that they did not participate," U.S. Secretary of State Hillary Rodham Clinton said at a news conference. "I expect that Pakistan will be involved going forward and we expect them to play a constructive role."
Participating nations jointly pledged political and financial long-term support to ensure Afghanistan's viability after international troops leave the country in 2014.
The U.S. announced it would free more than $650 million in support for small community-based development projects in Afghanistan, frozen because of financial irregularities in Afghanistan's key Kabul Bank.
Afghanistan estimates it will need outside contributions of roughly $10 billion in 2015 and onward. The country received $15.7 billion in aid this year, representing more than 90 percent of its public spending, it said.
Despite more than a decade of international intervention, Afghanistan still ranks among the world's poorest and most corrupt nations.
The Associated Press contributed to this report.