The attack took place at Tarinkot Airport in the the southern province of Uruzgan and was carried out by a member of the Afghan security forces, it said.
"An element of the progress is the capacity that we now have to believe that there is help, that numerous Taliban now see that they can't win on the ground militarily", stated the secretary of state.
The incident marks the first combat fatality for the US military in Afghanistan since April 30, and the third there this year.
"An American role will be important in this, but we can't run the peace talks, we can't settle this from the outside", he said.
The Afghan government expects the gathering to term the Taliban insurgency and use of suicide attacks in Afghanistan as unlawful, as similar meetings have recently in Kabul and in Jakarta.
Maciel was killed and two other American service members were wounded in what USA military officials called an "apparent insider attack" on July 7 while they were advising local forces at an Afghan military facility in the town of Tarin Kowt in Uruzgan Province. Two other Americans were wounded in the attack. "We're counting on all of the actors in the region to support this process and make sure the Afghan people can have their voices heard during these elections", he said. The relatively peaceful interlude yielded some reports of positive contact between Taliban fighters, Afghan government forces, and civilians, but also included two suicide bombings, one of which was claimed by the Islamic State.
In a statement, the Taliban said the ongoing jihad in Afghanistan was neither a war among Afghans or conflict among Muslims. "And I will tell you honestly, we have had our Afghan partners come to us with intelligence that preempted potential attacks, and they have been proactively taking care of their own problems".
His awards include the Bronze Star, Purple Heart, Army Achievement Medal, and Afghanistan Campaign Medal, officials said, according to news website Military.com.
Some of the U.S. forces are involved in counterterrorism operations, particularly against the Islamic State jihadist group.