US Defence Secretary Robert Gates has held talks with Afghan President Hamid Karzai in Kabul.
The talks focused on Washington's new strategy to send 30,000 extra troops to fight the Taliban and the training and retention of Afghanistan's army and police.
It is the first official US visit since President Barack Obama last week announced he would boost the US deployment in Afghanistan to 100,000 to counter the Taliban insurgency.
The Pentagon has said the first wave of 1,500 extra US Marines will begin arriving in southern Afghanistan next week.
However, President Karzai said that Afghanistan lacked the resources to fund its security forces for the next 15 to 20 years.
He said: 'For 15 to 20 years, Afghanistan will not be able to sustain a force of that nature and capability with its own resources.
'We hope that the international community and the United States, as our first ally, will help Afghanistan reach the ability to sustain a force.'
Mr Karzai faces huge pressure to form a transparent government after his fraud-tainted re-election in August.
He has postponed the unveiling of his long-awaited cabinet until Saturday at the earliest, a parliament spokesman said.
The US has warned Mr Karzai to fight corruption or see his cabinet bypassed in favour of lower level officials to provide services to Afghans as part of the sweeping new war strategy.
British Defence Secretary Bob Ainsworth has also arrived in Afghanistan on a surprise visit.
His visit comes as the British death toll this year in Afghanistan rises to 100.
Mr Ainsworth is in the troubled southern Helmand province, where he is due to meet British troops and Afghan officials. The visit is expected to last several days.
