The gunmen who attacked the policemen on Monday, were wearing military uniforms when they fired on police vehicles on a road in the Del Khak area of Farah, Interior Ministry spokesman Lutf Allah Mashal said.
Mashal said he did not know who had carried out the attack, but said it could have been the work of Taliban fighters or drug traffickers. "An investigation has been launched," he said.
In a separate incident on Sunday, US forces killed five Taliban suspects and wounded three in the southern province of Zabul, Zabul Governor Kheyal Mohammad Husseini told reporters.
Farah borders Iran, one of the main routes for the export of Afghanistan's opium and heroin output.
In April, seven policemen were killed by gunmen wearing military uniforms in another part of the province and authorities blamed remnants of the Taliban movement.
Farah has been regarded as one of more secure parts of Afghanistan since US-led forces overthrew the Taliban in late 2001.
The attack on the policemen follows attacks by suspected Taliban supporters on election workers and prospective voters in the elections in which at least 18 people have been killed since Friday.
"It could have been the work of Taliban fighters or drug traffickers. An investigation has been launched"
Lutf Allah Mashal
Interior Ministry spokesman
The news came as President Hamid Karzai headed for a NATO summit in Turkey to appeal for more NATO troops to protect the polls, although the number of soldiers offered is likely to fall well short of the 5000 or so extra soldiers he is asking for.
More than 800 people have been killed, most of them across southern and eastern Afghanistan, since August, largely in raids blamed on the Taliban and their allies.
The Zabul governor said five alleged Taliban fighters were killed near the provincial capital of Qalat after a group of fighters hiding in a village fired on a US helicopter.
There was no immediate comment from the US military.