Secretary of State John Kerry paid tribute to a female US diplomat who was among six NATO personnel killed in two attacks in Afghanistan on Saturday, the deadliest day for foreign forces this year.
Kerry, who had met the unnamed victim during a visit to Kabul last week, described her as "smart, capable, eager to serve and deeply committed to our country and the difference she was making for the Afghan people."
He offered his condolences to her relatives for their "immeasurable loss."
She was killed in an attack that also claimed the lives of three American soldiers, an Afghan civilian NATO worker and an unknown number of other Afghan civilians.
Kerry said the American officials and their Afghan colleagues were traveling to donate books to students at a school in Qalat, the capital of southern Zabul province, when they were killed in the "despicable attack."
The Zabul blast left four State Department staffers injured, one critically, according to Kerry.
An American citizen also died in "an insurgent attack" in eastern Afghanistan, US-forces Afghanistan said.
The strikes came as the top US military officer General Martin Dempsey arrived in the country on an unannounced visit, and raise troubling questions about the Taliban's strength ahead of the coalition's withdrawal in 2014.
"I wish everyone in our country could see first-hand the devotion, loyalty and amazingly hard and hazardous work our diplomats do on the front lines in the world's most dangerous places," Kerry said.
"Every day, we honor their courage and are grateful for their sacrifices, and today we do so with great sadness."
Kerry said he was in close contact with Defense Secretary Chuck Hagel, the White House and senior officials at the State Department about the attack.