The US has refused to ban air strikes and denied the use of white phosphorus in Afghanistan following the deaths of civilians in an attack last week.
Afghan President Hamid Karzai, citing the Red Cross, said up to 130 civilians may have been killed when air strikes struck crowded homes in two villages in Farah province.
On Saturday, the US military officially acknowledged that the air strikes in western Afghanistan took place.
If the death toll claimed by Mr Karzai is confirmed, it would be the biggest such case of Western forces killing civilians since they invaded in 2001.
As hundreds of Kabul university students marched yesterday in protest against the killings, White House National Security Advisor James Jones told US media that it would be 'imprudent' to tie the hands of commanders by banning air strikes.
But he promised the US would 'redouble efforts to make sure that innocent civilians are not killed'.
Hamid Karzai made his demand for an end to such attacks last week during a visit to Washington in which he met President Barack Obama.
Doctors examine 'strange burns'
Nader Nadery, a member of Afghanistan's independent human rights commission, said doctors who had treated victims from the incident had reported strange burns they believed may have been caused by a chemical like white phosphorus.
He said they 'had burns on bodies and face, and the doctors have said it was something not usual from a bomb explosion'.
White phosphorus, which erupts in flame on contact with air and can stick to flesh causing severe burns, is legal on the battlefield to create light or smoke and is not banned by treaties that forbid using chemicals as weapons.
But its use in populated areas to 'smoke out' enemy fighters has been a persistent source of controversy.
'There was no smoke or illum (illumination) used in Farah,' US military spokesman Colonel Greg Julian insisted.
Meanwhile suicide bombers on motorbikes killed at least seven people yesterday when they attacked a police convoy in Afghanistan's southern province of Helmand.
