The teenage girls fell ill and some became unconscious after smelling gas at their school in Ghazni, a two-hour drive south of the capital, Kabul, said senior provincial police officer Nawroz Ali Mahmoodzada.

"It is again the same kind of attack to discourage girls from attending schools," Mahmoodzada told Reuters. "It is very disturbing. We have not yet found any clues to say where this substance is from or who is behind it."

Safiullah, a doctor in Ghazni's central hospital, said most of the girls were treated and discharged. Others were still under medical care.

Saturday's incident followed a similar pattern of attacks on girls' schools involving an airborne substance which officials say could be poisonous gas. In other recent attacks in Kabul and in northern Kunduz province, girls reported smelling something sweet and then began fainting, and suffered dizziness and vomiting. However, none of those cases resulted in deaths or long-term health problems.

The Taleban, which banned education for girls during their rule from 1996-2001, has condemned such incidents in the past and denied any responsibility. They have, however, torched dozens of schools, threatened teachers and even attacked schoolgirls in rural parts of the country.