U.S. Secretary of State John Kerry has denounced as "despicable" a pair of Taliban attacks on April 6 which killed six Americans, including a young female diplomat.
Three U.S. soldiers, the diplomat, and a Defense Department employee were killed when they were struck by an explosion while traveling to deliver books to a school in Qalat, the capital of southern Zabul Province.
"The Washington Post" identified the diplomat as Anne Smedinghoff, 25. Smedinghoff was Kerry's embassy guide and aide when he visited Afghanistan last month, the paper said.
Kerry paid tribute to the young female diplomat, saying she had given her life "to give young Afghans the opportunity to have a better future."
A convoy carrying the governor of Zabul Province was also traveling on the same road and to the same event at the time of the explosion.
Provincial Governor Mohammad Ashraf Nasery survived the attack in Qalat.
Nasery told the Associated Press that he believed he was the intended target of the attack, apparently carried out by a suicide bomber.
The Taliban claimed responsibility for the attack.
In a separate incident, one American was killed in what the U.S.-led international coalition said was "an insurgent attack" in eastern Afghanistan.
It was the deadliest day for the United States in the war in eight months.
The attack came just days after insurgents stormed a courthouse, killing more than 46 people in one of the deadliest attacks of the war, now into its 12th year.
The latest bloodshed, commentators say, highlights the instability Afghanistan still faces as foreign combat troops prepare to withdraw by the end of 2014.
The violence came just hours after the top U.S. military commander arrived in Afghanistan for consultations with Afghan and U.S.-led coalition officials.
General Martin Dempsey was expected to discuss the rate at which U.S. forces could be withdrawn and the progress being made by U.S.-led forces in training Afghan security personnel.
With reporting by AP and Reuters