The drops scatter packets over a wide area
Jean Ziegler, whose job is to defend the right to food, described the US actions as "totally catastrophic for humanitarian aid".
Separately, the UN children's agency Unicef warned on Monday that 100,000 Afghan children could die this winter unless food reaches them before the end of the year.
Mr Ziegler criticised the aid distribution strategy known as "snowdropping" because it involves scattering food over a wide area.
Condemnation
"I must condemn with the last ounce of energy this operation called snowdropping," he said in Geneva on Monday.
He warned that dropping food over wide areas could send hungry children into mine fields.
He also said combining military and humanitarian missions could endanger the lives of aid workers, the BBC's Emma Jane Kirby says.
"If you do not separate very clearly military operations and humanitarian operations, you destroy totally the credibility of the humanitarian operations," Mr Ziegler said.
He called on the US to end its military campaign against Afghan targets.
'Food imperialism'
There have also been reports that Afghans picking up the packets are unable to read the instructions printed in English which explain they contain enough food for 24 hours.
The locals select the few items that interest them and leave the remainder to rot.
There are also reports that not all Afghans appreciate the taste of peanut butter - included in the packs along with strawberry jam.
"Yuk! A local man will never eat that!" an Afghan soldier in Khoja Bahawuddin is quoted as saying by Le Monde newspaper.
US Republican congressman Jim Colby, who chairs the House of Representatives' committee which approves the funds for aid, has said that the aid drops are at best a gesture.
WFP support
But another UN official, Christiane Berthiaume of the World Food Programme, said the problem of Afghan hunger was so severe that any means of easing it must be welcomed.
"Food at this time, from wherever it comes - it's food for hungry people."
The people of Afghanistan were already facing famine long before the US and UK began military operations against the country in the wake of the suicide attacks on New York and Washington.
Sanctions against the Taleban regime, combined with drought and limited UN resources to combat hunger, led to warnings a year ago that millions of people in South Asia could starve.
Some aid convoys have reached Afghanistan by road from Pakistan and Iran since the air strikes began on 7 October.