A wave of bomb blasts and shootings killed 32 people, including two children, in Afghanistan Monday as President Hamid Karzai jetted to Washington for a summit on defeating extremism.

In the bloodiest incident a bomb tore through a tractor and trailer transporting a group of Kuchi nomads in a remote district of southern Zabul province, a district governor said.

"A roadside bomb struck their vehicle and killed 12 people," said Wazir Mohammad, head of the Shamalzai district on the border with Pakistan.

The dead were six women, two children and four men, he said.

There was no claim of responsibility for the blast but there are regular bombings in Afghanistan, most of them blamed on Taliban fighters targeting Afghan and foreign security forces.

Mohammad also said two Taliban fighters were killed in the same district early Monday when mines they were planting in a road exploded.

Taliban fighters elsewhere attacked a construction site near the Zabul capital, Qalat, where labourers were rebuilding bridges destroyed in earlier militant action, deputy provincial police chief Ghulam Jialani said.

Six security guards protecting the workers were killed in the shoot-out, along with two civilian passers-by, he said.