Ban said security concerns - a reference to continued fighting in the south - prevented him from widening the UN presence in the country.
"In order to carry out such efforts, we need a reasonable level of freedom of movement and security," he said according to a text of his speech made available by UN officials.
Tom Koenigs, UN Afghanistan envoy, said that he expected Afghanistan's neighbours and key Nato states to backup their pledges to Afghanistan.
"If I expect one thing to come out of this meeting, it is that they reinforce the commitment to Afghanistan," he said.
"We need more troops, we need more money and we need a sustainable commitment in Afghanistan."
The 24 participants at the UN are members of a board established to implement an "Afghanistan Compact", a five-year blueprint adopted by the Afghan government in January 2006 to help the nation build a stable future after two decades of war.
The agenda sets out benchmarks to promote security, good governance, the rule of law, human rights and economic and social development in Afghanistan as well as to fight the drug trade.
Diplomats, however, said that Sunday's meeting was not expected to result in specific gains.
Western countries have been pressing for the UN to boost its profile in Afghanistan after Koenigs quits at the end of 2007, with a high-level mission chief who could act as a "partner" for Karzai.
Koenigs said the Taliban could not be defeated by military means alone.
"There must be a comprehensive strategy which comprises civilian and military action, so we come to a political offensive against the insurgency," he said.
Last week, an Afghan presidential spokesman said Kabul was ready for peace talks with the Taliban but would not accept their preconditions, such as the withdrawal of all foreign troops.
The Afghan meeting is the latest of several Ban has convened to underscore the central UN role.
About 50,000 foreign troops are deployed in Afghanistan, including a Nato-led International Security Assistance Force (Isaf), and separately led US forces.
A UN mission supports and advises the Afghan authorities on economic and political development, justice reform, humanitarian aid and anti-drug programs along side Nato and a separate US military presence.