The NATO Resolute Support Mission in Afghanistan said the two service members were killed on Wednesday.
Their names were being withheld until after their relatives were notified, the statement said.
The deaths bring to at least 14 the number of US military personnel to be killed in action in Afghanistan this year.
The dialogue is aimed at ending 18 years of military intervention, launched after the September 11, 2001, attacks on the US.
A breakthrough could pave the way for a withdrawal of international troops from Afghanistan, including some 14,000 US soldiers.
The troop pullout is a key Taliban demand. Washington, in turn, wants guarantees the country would not revert to being a launchpad for global attacks.
After the Doha talks, Khalilzad will need to lay the groundwork for peace talks between the Taliban and the Afghan government, which would come after any deal with Washington.
US President Donald Trump, a vocal critic of the Afghan war before he was elected in 2016, said US forces have been stuck there acting like a “police force”.
He is believed to want to see a withdrawal of most of the US troops in Afghanistan before November 2020, when he will seek re-election.
But the Republican leader has acknowledged that some of those US forces would have to stay.
“We have to have a presence, yes. The Taliban does not respect the Afghan government,” Trump said earlier this week, adding: “It is a dangerous place and we have to keep an eye on it.”