The bomb tore through a police vehicle in Herat, a relatively peaceful province, provincial police spokesman Abdul Rauf Ahmadi told AFP.
"A mine planted by the terrorists hit a police vehicle in Guzara district yesterday morning, leaving six policeman dead and three more wounded," Ahmadi said.
The toll was confirmed by the provincial governor's spokesman Ehsanullah Hayat.
There was no immediate claim of responsibility but roadside bombs have been the Taliban's weapon of choice in their war against foreign and Afghan security forces.
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The blast comes days after Afghan Taliban leader Mullah Omar issued a statement endorsing as "legitimate" a nascent negotiation with the Afghan government representatives aimed at ending the conflict.
Earlier in the month, neighbouring Pakistan hosted the first face-to-face talks between the Afghan government and the Taliban, reportedly supervised by US and Chinese representatives.
The meeting was said to have made progress, with the two sides agreeing to work on trust-building measures and hold more such sessions after Eid.
NATO ended its combat mission in Afghanistan in December, leaving local forces to battle the Taliban alone, but a residual force remains for training and counter-terrorism operations.