Fischer called on rich nations to provide massive aid
As the deal was signed near Bonn, aid donors who could approve speedy emergency assistance met in Berlin.
"This conference must give a clear signal that we will not leave the Afghan people alone in their hunger and need," German Foreign Minister Joschka Fischer said at the opening ceremony in the German capital.
Afghanistan needs massive long-term international help with reconstruction, he said. There is an urgent need to improve transport routes allowing aid into the country and to clear mines allowing agriculture to resume.
Germany puts the cost of reconstructing Afghanistan at about $6 billion, although an official United Nations estimate has yet to be reached.
Mark Malloch Brown of the UN Development Programme said the process would be taken in three phases: relief, recovery and reconstruction.
There is currently only enough funding for 60% of the relief phase, he said. The most expensive phases, recovery and reconstruction, have yet to be funded at all.
But he stressed the importance of raising the money now, before political attention turns away from Afghanistan.
European aid
The European Union welcomed the Bonn accord with an immediate promise of help.
External Affairs Commissioner Chris Patten said the 15 EU countries were working on an "initial package of support to the interim administration to help it to address the formidable challenges it faces".
The German foreign office says $1.3 billion has already been raised by the nations of the Afghan support group plus the EU and the UN.
The World Food Programme of the UN estimates it would take at least 52,000 tonnes of food per month to feed an estimated six million needy Afghans.
The United Nations Development Programme has drawn up reconstruction guidelines spanning a period of five to eight years.
Rebuilding Afghanistan's shattered cities will be high on the list.
Other priorities are roads, agriculture, water, sewage, de-mining, health and education.
Farmers
Agriculture has been severely disrupted, with a vast number of Afghans earning a living by growing opium poppies.
The UN plans to offer them "alternative livelihood strategies".
Aid agencies plan to provide farmers with seeds and help with irrigation.
But Mr Malloch Brown says the ultimate aim is self-sufficiency.
Afghan farmers have to become independent of international aid, just as the country must develop an autonomous system of justice, security and revenue collection, he says.
The reconstruction effort is to be financed through an external trust fund run by foreign development and financial experts, under a World Bank plan.
Money might be channelled through an independent Afghan reconstruction agency run by a combination of foreign and Afghan experts. This would provide a safeguard against corruption, in-fighting and waste.
With no central bank or treasury and no formal system of revenue collection, the reconstruction task is huge, experts say. But Afghanistan does have huge economic potential with gas and oil reserves.
Very little works in Afghanistan. Electricity is unreliable, many roads are unusable, and municipal water systems have been destroyed.
Less than a quarter of the population has access to safe water and only 12% of people have access to adequate sanitation.
The education system is also in crisis.
Only 39% of boys and 3% of girls attend any form of primary school, according to UNDP figures.