(CNN) -- The death toll has surged from weekend violence in Afghanistan, with six Americans and more than a dozen Afghan civilians reportedly among the casualties.
Five Americans -- including a U.S. diplomat, a civilian from the Defense Department and three U.S. service members -- were killed while delivering books to an Afghan school, when a suicide bomber struck their convoy in southern Afghanistan's Zabul province Saturday. Another U.S. service member was killed in a separate attack Saturday.
The diplomat's death was a grim reminder of the risks and importance of pushing for change in "one of the toughest places on earth," U.S. Secretary of State John Kerry said.
Anne Smedinghoff, 25, is believed to be the first U.S. diplomat killed since a September attack in Benghazi, Libya.
"I think there are no words for anybody to describe the extraordinarily harsh contradiction of a young 25-year-old woman with all of the future ahead of her, believing in the possibilities of diplomacy, of changing people's lives, of making a difference, having an impact, who was taking knowledge in books to deliver them to a school," Kerry said.
In separate clashes in Kunar province on Saturday, local officials said, at least 11 children were killed in a NATO operation aimed at Taliban targets. NATO officials said they were investigating reports of civilian casualties.
Family: Diplomat "was doing what she loved"
Kerry was emotional Sunday as he spoke to U.S. diplomats stationed in Istanbul about Smedinghoff's death.
Smedinghoff was "vivacious, smart, capable, chosen often by the ambassador there to be the lead person because of her capabilities," Kerry said.
The attack killed Afghan civilians and wounded four more State Department personnel. One of them was in critical condition in a hospital in Kandahar, Kerry told reporters Sunday.
"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 in a statement Saturday. "Every day, we honor their courage and are grateful for their sacrifices, and today we do so with great sadness."
Smedinghoff volunteered for an assignment at the U.S. Embassy in Kabul and had been working there since July, her parents said in a statement.
"She particularly enjoyed the opportunity to work directly with the Afghan people and was always looking for opportunities to reach out and help to make a difference in the lives of those living in a country ravaged by war," Tom and Mary Beth Smedinghoff said. "We are consoled knowing that she was doing what she loved, and that she was serving her country by helping to make a positive difference in the world."
Afghan children killed
At least 11 Afghan children were killed Saturday in a joint military operation by Afghan and NATO forces, local officials said.
Mohammed Zaher Sapai, governor of the Shigal district, said 11 children were among 18 people killed in the operation.
NATO's International Security Assistance Force is investigating, spokesman John Manley said.
"We take these reports of civilian injuries very seriously," he said.
U.S. military brass visits
The chairman of the U.S. Joint Chiefs of Staff, Gen. Martin Dempsey, is in Afghanistan for meetings with coalition and Afghan leaders.
Dempsey will be briefed on the ongoing military transition, where Afghan security forces are taking greater control of their country.
He will meet with Marine Corps Gen. Joseph F. Dunford, who took command of ISAF about two months ago.
"One of the things I'm going to ask (Dunford) is ... what have you learned," Dempsey told the American Forces Press Service.
"If he tells me 'Nothing,' I'm going to say, 'Maybe we got the wrong guy.'"
Dempsey spokesman Col. David Lapan later told CNN that the chairman was simply joking when he made the remark about Dunford. Dempsey has total confidence in Dunford, Lapan said.
CNN's Michael Martinez, Jill Dougherty, Jamie Crawford, Chris Lawrence and Barbara Starr contributed to this report.