There also were a number of Taliban casualties in the Zawol district ambush.
The group warned its fighters would "strongly defend" themselves if attacked.
Antonio Guterres urged the Taliban to observe the ceasefire declared by the Afghan government.
It was not immediately clear if the Taliban had agreed to the ceasefire.
US and NATO commander in Afghanistan disclosed that Taliban, Afghan officials in ceasefire talks.
The bomber detonated an explosive-laden vehicle at the entrance of the police station.
The latest fighting in Farah came a day after officials said the city was clear of insurgents.
The defence ministry initially denied civilians had been hurt, then claimed Taliban had shot them
The US carried out more drone strikes overnight and the Afghan army is still clearing the city.
Ten people were killed when militants detonated bombs and stormed a government building in Jalalabad
An explosion at a mosque in the eastern Afghan province of Khost on Sunday killed 17 people
With the help of US air forces, the Taliban militants have been pushed back
Taliban said offensive was partly a response to US President Trump's new strategy for Afghanistan.
Ministers from 80 countries gather in Paris for conference on combating financing of terror groups.
'I miss Pakistan every day'
Taliban attacks overnight in Afghanistan killed at least 13 policemen and wounded several others.
The total number of civilian casualties in the January-March period reached 2,258.
Provincial governor Lutfullah Azizi confirmed Hekmat had been killed.
The airstrike killed or wounded dozens of people, many of them children.
He was addressing rally in Garhi Khuda Baskhsh.