The attack occurred on Tuesday as the police chief of Dishu district in Helmand province was travelling with two police vehicles and a group of bodyguards, Dishu mayor Muhammed Rahim said.
Aljazeera's correspondent in Afghanistan reported six Afghan security men and Taliban armed fighters were killed during the ambush.
The police chief was unhurt.
Police immobilised one of the assailants' cars with rockets but failed to prevent their escape in other vehicles.
Rahim said it was unclear whether the attackers, who he asserted were Taliban insurgents, were hurt in the exchange.
An official said on Wednesday that three civilians were hurt when US troops shaken by a roadside bomb blast in eastern Afghanistan opened fire on an approaching bus.
The bomb exploded near US military vehicles travelling through Khogyani district of Nangarhar province on Tuesday, Faizan ul-Haq, a spokesman for the provincial government said.
The Taliban has undermined
claims of improved security
"But by misunderstanding, they opened fire on a small bus coming toward them," ul-Haq said.
He said that two men and one woman among the passengers were hurt and that US troops took all three away for treatment.
A witness, Delsoz Khogyani, said the incident happened in the village of Karem Khel.
"It's a busy road and the coach was coming from Jalalabad with 18 people on board and the Americans fired their guns at it," Khogyani, a farmer, said.
He said people in the village believed the three victims had been killed and were "very upset that they took their bodies away".
US military spokeswoman Lieutenant Cindy Moore said in Kabul the civilians were probably caught in the crossfire after a group of fighters attacked a coalition patrolling unit.
"The investigation will determine whether they were hit by coalition or insurgents," she said, adding that fighters took cover behind the passing civilian vehicle after attacking coalition soldiers.
She said prior to the gun-fight fighters detonated two improvised bombs without causing any US casualties.
US soldier killed
A US soldier was killed when attackers opened fire on his unit during a patrol in central Uruzgan province, the military said on Wednesday.
A US soldier was killed after
attackers fired on his patrol
The soldier was evacuated for emergency medical treatment to a US base after the attack on Tuesday in Deh Rahwood, 400km southwest of the capital, Kabul, but died.
His remains were sent to a US airbase in the southern city of Kandahar, for transportation back to the US, the military said in a statement.
The soldier's name was not released.
Taliban-led fighters have recently stepped up their three-year uprising across the south and east of the country, undermining assertions from American commanders and Afghan officials that security has improved significantly.