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At least three teachers have died and 15 more people have been wounded after a roadside bomb hit a bus carrying university staff in Afghanistan.

Professors and staff from Al-Biruni university were travelling through Charikar, the capital of Parwan province, about 70km (43 miles) north of the capital Kabul.

No group has claimed responsibility.

Afghanistan has seen increasing violence in recent weeks as the US and Nato prepare to withdraw all troops.

The deadline for the withdrawal is