Taliban militants have killed six Americans, including a young female diplomat, in the deadliest day in Afghanistan for the US in eight months.
Three members of the military, two US civilians and an Afghan doctor died after being hit by an explosion while travelling to donate books to a school in the south.
In a statement, Secretary of State John Kerry paid tribute to Anne Smedinghoff, a 25-year-old foreign service officer with the US State Department, who was killed in the "despicable attack" in the province of Zabul.
He said: "She tragically gave her young life working to give young Afghans the opportunity to have a better future."
The Illinois woman is the first American diplomat to die on the job since last year's attack on the US diplomatic mission in Benghazi, Libya.
"We also honour the US troops and Department of Defence civilian who lost their lives, and the Afghan civilians who were killed today as they worked to improve the nation they love," Mr Kerry added.
Officials said the suicide car bomber struck just as a coalition convoy drove past a caravan of vehicles carrying the governor of Zabul to the same event.
Another American civilian was killed in a separate insurgent attack in eastern Afghanistan, the US military said in a statement.
It was the deadliest day for Americans since August 16 2012, when seven American service members were killed in two attacks in Kandahar province, the birthplace of the Taliban insurgency.
Six were killed when their helicopter was shot down by insurgents and one soldier died in a roadside bomb explosion.
Saturday's violence came hours after the US military's top officer General Martin Dempsey landed in Afghanistan for talks with local and coalition officials as foreign forces work to pull nearly all of their combat troops out of the country by the end of 2014.
Taliban spokesman Qari Yousef Ahmadi claimed responsibility for the attack in Zabul and said the bomber was seeking to target either a coalition convoy or the governor.
"We were waiting for one of them," Ahmadi said in a telephone interview with The Associated Press. "It was our good luck that both appeared at the same time."
Insurgents have stepped up attacks around the country in recent weeks as Afghanistan enters what could be one of the most critical periods following the US invasion in late 2001 that ousted the Taliban.
The majority of US and coalition forces are expected to begin a significant drawdown in the latter part of this year, leaving Afghan forces in charge of security across the country within months.
Afghanistan is also gearing up for a presidential election next spring, and the Taliban have not yet accepted an offer to engage in peace talks in the Gulf state of Qatar.
There are about 100,000 international troops in Afghanistan, including some 66,000 from the US and around 9,000 from Britain.
The US troop total is scheduled to drop to about 32,000 by early next year, with the bulk of the decline occurring during the winter months.
British troop numbers are expected to fall from 9,000 to around 5,200 by the end of 2013.
While there has been no final decision on the size of the post-2014 force, US and Nato leaders say they are considering a range of between 8,000 and 12,000 - most of them trainers and advisers.