Police spokesman Abdul Raouf Ahmadi said the first blast hit a civilian vehicle in Badghis province, killing 10 passengers. The second, minutes later nearby, struck a police car, killing two policemen.
The strikes took place on Sunday, but were not reported until Monday morning.
Badghis, in the northwest of Afghanistan, is one of the northern areas that has seen increasing militant activity as Taleban fighters spread their influence from traditional strongholds in the south and east of the country.
Roadside bombs are by far the militants' most lethal weapon, usually targetting police or government troops as well as foreign forces. Frequently, civilians are also killed.
Civilian deaths caused by Western or government forces are a source of intense anger in Afghanistan, but the United Nations says most civilian deaths are caused by insurgents and the number killed by troops is declining.
Mullah Abdul Manan, a militant commander in the area, said the two bombs had killed Afghan and foreign troops, and denied that civilians had been killed.
