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.