His comments came after Afghan officials said 25 civilians were killed in an alliance air strike in southern Helmand Province.
De Hoop Scheffer said the June 21 incident showed the alliance needed to continually look at its rules of engagement.
He also blamed the Taliban for using civilians as human shields.
RFE/RL correspondent Salihmohamad Sali reported from Afghanistan's southern Helmand Province that the air strike targeted a group of homes in Helmand's Gereshk district where Taliban fighters were firing on foreign and Afghan forces.
Around 20 suspected Taliban were also killed in the air strike, which came just as President Hamid Karzai urged international forces to do more to prevent civilian casualties.
Also today, Dutch military chief General Dick Berlijn said Taliban fighters executed civilians who refused to fight alongside them during a battle earlier this month.
Berlijn said Dutch and Afghan forces engaged Taliban fighters in the southern town of Chor about a week ago. He said Taliban militants tried to force civilians to fight for them and killed those who refused.
(RFE/RL's Radio Free Afghanistan, Reuters, AFP)
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