Shah Poor Ahmadzai, the provincial police chief's spokesman, says seven others were wounded in the attack late Friday, which took place in Mohammad Agha district.
This is the second time in four months that American officials have engaged in direct talks with Taliban representatives.
"We can't confirm specific meetings or the content of diplomatic conversations", said a State Department spokesperson, speaking on the condition of anonymity.
Farah provincial council said Sunday that a group of Taliban insurgents attacked several security check posts in the province.
Last week, Khalilzad flew into Kabul to discuss measures that could set the wheels of negotiations in motion, by trying to bring President Ashraf Ghani's government and the Taliban on the same table. However, despite the move, the Taliban have continued to gain ground in the country, questioning the effectiveness of the strategy and US' role in the process.
He said the Taliban also suffered casualties. During Khalilzad's meeting with government leaders last week, the Taliban - breaking months of silence - had warned against the elections, with threats to derail it.
Khalilzad's trip started in Afghanistan and he travelled to Pakistan, the United Arab Emirates, Saudi Arabia and Qatar before returning to Kabul.
Last week, Taliban insurgents issued a statement condemning the elections and warning candidates and Afghan Security Forces that they would be targets if they participate.
The Office of the President, ARG Palace, in a statement said Ambassador Khalilzad presented a report regarding his recent trips to the regional countries and emphasized that the United States is prepared for an all-out support to an Afghan-led peace process and efforts are underway in this regard.
"The United States shares the aspirations of all Afghans for a peaceful Afghanistan where all Afghans see themselves included".
However, Tahir Khan said he believed the Taliban were not ready to sit down with the Afghan government as first they want to know when the US will withdraw its forces from Afghanistan. "All citizens of Afghanistan must be a part of this reconciliation process", said Khalilzad, according to the embassy statement at the end of his four-nation tour.
The Taliban had reportedly met with US officials in July after an unprecedented ceasefire in June which fueled hopes that negotiations could bring an end to fighting.
They have ramped up attacks in strategic provinces and have also directed Afghans to boycott parliamentary elections scheduled for October 20.
At least 8,050 Afghan civilians were killed or wounded in the first nine months of 2018, nearly half of them targeted by suicide bomb attacks and other improvised devices that may amount to war crimes, the United Nations said last week.