Qasem Khan, a spokesman for the province's governor said three police officers were killed and two others wounded.
Khan said no Nato force personnel were wounded. Nato officials could not immediately be reached for comment, however.
Another remote-controlled bomb in western Herat province killed four civilians - all men who were riding bicycles or walking in the area - and wounded six people, including border police commander Mohammad Ayub and his bodyguard, Noor Khan Nikzad, Herat police spokesman, said.
The bomb exploded near Herat airport just as Ayub's convoy was passing by, Nikzad said.
Separately, the Afghan army captured four suspected Taliban fighters on Wednesday in the southwestern Farah province, the defence ministry said in a statement on Thursday.
There were no details on the suspects. This year has seen a surge in fighting. About 4,000 people, mostly Taliban fighters, have died during 2006, the bloodiest period in Afghanistan since the fall of the Taliban government in late 2001.
Military aide held
In another development, a close aide to the British commander of Nato troops in Afghanistan has been accused of passing secrets about activities there to Iran, press reports said on Thursday.
The reports come as British-led forces intensify their battle in the south of the country, invaded by US-led forces following the September 11, 2001, attacks.
Corporal Daniel James, an interpreter to Lieutenant-General David Richards, the head of Nato's more than 30,000-strong force in Afghanistan, has been charged under the Official Secrets Act with "prejudicing the safety of the state".
Specifically he is accused of passing information "calculated to be directly or indirectly useful to the enemy" by communicating with a "foreign power", believed to be Iran, The Daily Telegraph and other newspapers said.
James, 44, appeared at London's City of Westminster magistrates' court on Wednesday, but details of the case were not revealed as the judge ordered reporters out of court because of a "possible prejudice to national security".