KABUL, Afghanistan — A suicide car bomb in southern Afghanistan killed three American soldiers and two American civilians, including a State Department Foreign Service officer, on Saturday, the deadliest single attack against United States forces this year, officials said.

The violence came as Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, made an unannounced visit to Afghanistan.

Attacks are picking up in what is known as the country’s fighting season as the weather gets warmer. And the Taliban are expected to intensify their efforts to destabilize the Afghan security forces as the NATO troops who have secured the country for the last decade start packing up for their departure at the end of 2014.

The suicide bomber detonated his vehicle on a road in Zabul Province around 11 a.m. as an American convoy passed by, according to American and Afghan officials. The officials and soldiers in the convoy were accompanying Gov. Mohammad Ashraf Nasery of Zabul Province on a trip to inaugurate a new school in Qalat, the provincial capital, where they were to deliver donated books.

Four other State Department employees were wounded in the blast, one critically. Three Afghans were also killed — a doctor accompanying the governor and two of Mr. Nasery’s bodyguards, said Muhammad Jan Rasoolyar, the deputy governor.

The United States government did not immediately identify the victims, but they included a Department of Defense employee, three service members and the Foreign Service officer.

Secretary of State John Kerry, who was traveling to Istanbul on Saturday, said that he had met the State Department officer during his visit late last month to Kabul. She had been assigned to provide him logistical support during his trip.

“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,” Mr. Kerry said in a statement in strikingly personal terms, calling the officer’s death an “immeasurable loss.”

Mr. Kerry said that he had spoken to the woman’s parents. “As a father of two daughters,” he said, “I can’t imagine what her family is feeling today, or her friends and colleagues.”

The Taliban claimed responsibility for Saturday’s blast, which occurred near a coalition base. It was unclear whether the attack was aimed at the coalition forces or the governor, who survived. The bombing was the deadliest for Americans since July, when a bomb in Wardak Province killed six United States service members, American officials said.

An American civilian was also killed on Saturday in a separate insurgent strike in the east of the country, although officials did not specify the nature of the attack. So far this year, 30 coalition troops have died in Afghanistan, 24 of them Americans, according toIcasualties, a Web site that tracks military fatalities.

The death tolls for coalition forces and Afghans have reversed as Afghans have taken the lead in battle, with Afghans now making up the vast majority of victims of combat-related deaths.

The attacks on Saturday came on the heels of a major Taliban assault in Farah Province last week that killed at least 44 people. In that strike, insurgents stormed a government compound, setting off a seven-hour gun battle that wounded more than 100 people. That attack highlighted the deteriorating security situation in Farah, a restive province that borders Iran to the west and where violent attacks in general have been on the rise.

Officials from Farah said insurgents have targeted the province as the government in Kabul has dealt with more violent areas of the country.

General Dempsey’s trip to Afghanistan on Saturday was intended as an assessment of training that the United States can provide to Afghan troops. During a visit to Afghanistan by the general in August, insurgents hiding outside the heavily fortified Bagram Air Base fired a pair of rockets that damaged his plane and slightly wounded two service members. The general was in his quarters and asleep at the time, his spokesman said.