No group has said it carried out the attack. Last week gunmen shot dead 35 highway workers in Paktia province.

Road workers are frequently targeted by Taliban militants.

The Taliban recently declared a "spring offensive" of attacks. This is the fourth attack in as many days.

The labourers, who worked for a local construction firm called Nisa, were on their way to work when the blast went off in rural Panjwayi district of Kandahar province.

According to the health director of Kandahar, Abdul Qayum Khan, 18 of the 28 injured are in critical condition.

This attack took place only weeks after Nato forces conducted operations in the district, allowing for the reconstruction of roads and much-needed irrigation canals.