Sayed Agha Saqib, the police chief of the southwestern province of Farah, said: "They were definitely Taliban. No one can carry out such a cowardly ambush except the Taliban."
Suspected Taliban also assassinated a district police chief in neighbouring Nimroz province, killing three of his bodyguards. Three attackers were also killed, police said.
The Taliban and other insurgent and criminal groups have been stepping up attacks on Afghan and foreign forces, plunging the country into its bloodiest period since the Islamist group was toppled in late 2001.
Locals around the southern city of Kandahar, the birthplace of the Taliban, say fighters are asking them to flee their homes ahead of a possible operation by foreign troops.
In Loi Karez district, close to the border with Pakistan, armed Taliban fighters have been coming down from the mountains for the past few days telling residents to leave, said villager Nek Muhammad.
The Taliban is telling people to seek shelter in Pakistan and warning they may be forcibly moved if they ignore the warnings.
US-led forces recently launched a major operation in the south to quash the Taliban and Nato has stepped up its own operations to clear areas and drive out the fighters.
Afghan officials in Kandahar would not comment on the reported Taliban warnings.
Nato announced later on Saturday it has launched Operation Medusa to drive Taliban fighters out of Kandahar.
"The goal is to remove the Taliban threat ... and stabilise the district so that much-needed reconstruction and development projects can resume," Nato said.
About 2,000 people, most of them insurgents but also including civilians, Afghan forces, aid workers and more than 90 foreign soldiers, have been killed in violence this year.
Violence this year has claimed
about 2,000 lives in Afghanistan
The violence involves a mixture of opposition to foreign and government forces, tribal wars, the illegal drugs trade and crime.
The insurgency is concentrated in the south and east, mostly in provinces bordering Pakistan, the Taliban's one-time backer.
A British soldier was killed and another badly wounded in an attack by fighters in the southern province of Helmand, the main opium-growing area, on Friday, Nato said in a statement.
"The goal is to remove the Taliban threat ... and stabilise the district so that much-needed reconstruction and development projects can resume."
He was the seventh British soldier killed in fighting in Helmand since the beginning of August, when Nato formally took over southern Afghanistan from US soldiers to allow Washington to scale back.
Britain has faced unexpectedly fierce resistance from Taliban fighters since sending the first large foreign force to Helmand this year as part of an expanding Nato peacekeeping mission in the alliance's biggest ever ground operation.
A senior British commander said last week Britain's troops in the south were pulling back from mountain redoubts to focus on safeguarding reconstruction in lowland valleys.