The UN secretary-general's special representative in Kabul, Tom Koenigs, said that such disregard for "innocent lives" was staggering.

'A mistake'

He said there had been heavy violation of international humanitarian laws and that there could be no excuse for this "mass murder".

Mr Koenigs said the UN mission had repeatedly stressed the need for all sides to prevent civilian casualties.

Many civilians have also been killed in actions by international and Afghan government forces in recent months.

They have repeatedly said such deaths are a mistake.

Reports quote a Taleban rebel spokesman as saying the attack in Uruzgan was "bravely carried out by one of its fighters".

Isaf described the attack as inhuman and despicable.

Reports said many children were caught in the explosion as they were leaving classes near a crowded bazaar in Dehrawood.

Less than a month ago, another suicide attack in the same province killed at least 10 people including a Dutch soldier.

More than 6,000 people have died in insurgency-related violence in the past 18 months.

Uruzgan was also the scene of heavy fighting last month between Afghan and foreign forces on the one hand and Taleban rebels on the other.