KABUL, Afghanistan -- Five Americans were killed when a bomb targeted a convoy in southern Afghanistan on Saturday in the deadliest single combat incident for U.S. citizens this year.

In a statement, U.S. Secretary of State John Kerry described the dead as “an exceptional young Foreign Service officer,” a “Department of Defense civilian,” and “service members.”

In a separate statement, NATO confirmed that three soldiers had died, but did not identify them by their nationality. Kerry, however, noted that “We also honor the U.S. troops . . . who lost their lives.” Americans are the primary international military force in Zabul province, where the bombing took place.

NATO on Saturday also announced the death of a sixth American, a civilian worker who was killed in eastern Afghanistan. That made the day’s U.S. toll equal to the total number of American combat deaths that the Pentagon announced in March.

Four other State Department employees were wounded in the explosion, Kerry said, including one who was in critical condition.

The explosion also killed two Afghan civilians and narrowly missed the Zabul governor, Mohammad Ashraf Nasert.

According to the province’s spokesman, Sharif Nasiri, the attack took place at about 11 a.m. in the provincial capital, Qalat, when a suicide bomber detonated as the governor’s convoy passed the headquarters of the local NATO provincial reconstruction team on its way to the dedication of a school near the NATO base.

Kerry said the Americans had been targeted by “an IED” or improvised explosive device, as they were on their way to donate books to the school.

“Just last week in Kabul, I met our fallen officer when she was selected to support me during my visit to Afghanistan,” Kerry said. “ She was everything a Foreign Service Officer should be: smart, capable, eager to serve, and deeply committed to our country and the difference she was making for the Afghan people. She tragically gave her young life working to give young Afghans the opportunity to have a better future.”

Kerry did not release the woman’s name. He said he had spoken to her parents “and offered what little comfort I can for their immeasurable loss.”

He said U.S. officials were still investigating the details of the bombing.

The past winter had been an unusually quiet one in Afghanistan for international troops. But with the return of warm weather, insurgents have been stepping up attacks. In addition to the six announced combat deaths in March, five Americans died in a March 11 helicopter crash that the Pentagon says is still under investigation. Four other Americans died of causes not related to combat, according to Pentagon casualty reports. No combat deaths had been reported in the first five days of April.

On Wednesday, insurgents attacked a courthouse and adjacent buildings in western Afghanistan, leaving 44 civilians, soldiers and police officers dead. None of the casualties were coalition forces, however.

The two dead Afghans were a doctor who worked at a hospital that was also hit by the explosion and a student, spokesman Nasiri said. The NATO base, hospital and school are adjacent to one another.

One of the governor’s bodyguards and two Afghan civilians also were injured and taken to the hospital for treatment, Nasiri said.

The attack coincided with the unannounced arrival of Army Gen. Martin Dempsey, the chairman of the U.S. Joint Chiefs of Staff, who came to Afghanistan to assess what training Afghan forces will need after the combat role of the U.S.-led NATO force ends in 2014. That determination will help set the number of American troops that the United States will assign to Afghanistan after 2014. Dempsey was not near the attack.

The Taliban claimed credit for the Zabul bombing on their website. The website said that 13 foreign troops were killed, but Taliban counts of coalition casualties are often exaggerated.

Provincial reconstruction teams – widely called PRTs – were established by the U.S.-led coalitions in Afghanistan and Iraq to help local governments rebuild. They include a mix of troops, diplomats and experts on topics such as agriculture, civil engineering, law enforcement and the various functions of local governments.

Zabul is among the most restive provinces along the Pakistan border, though it hasn’t seen as much fighting as its neighbor to the south, Kandahar. The province is home to several U.S. bases, including FOB Apache, which Americans from the 555th Engineer Brigade from Joint Base Lewis-McChord near Tacoma, Wash., have been expanding in recent weeks to accommodate troops displaced by the closing or reduction of other bases as part of the U.S. drawdown of forces.

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