The bomb hit the vehicle of district governor Abdul Jabar on Saturday in Charchino, Uruzgan province, about 370km (230 miles) southwest of Kabul, Aljazeera's correspondent in Kabul said.
An Afghan official confirmed the attack but said only Abdul Jabar and two of his guards died. Three other security staff were wounded, said provincial governor Jan Mohammed.
"The Charchino governor and two of his bodyguards were killed and three other guards were wounded today after his vehicle was blown up by a remote-controlled bomb," he said.
The discrepancy in the tolls could not immediately be resolved.
A purported Taliban spokesman, Abdul Latif Hakimi, claimed responsibility for the attack in a telephone call from an undisclosed location.
"We claim responsibility for the attack on the district governor," he said. "Our mujahidin (holy warriors) planted the device for him and killed him."
Fighters loyal to the ousted Taliban government have stepped up their attacks against Afghan and US forces in the province in recent months ahead of scheduled national parliamentary elections in September.
On Saturday morning, two US soldiers and their Afghan interpreter were wounded in another remote-controlled roadside bomb blast, in the Deh Rawood district of Uruzgan, the US military said in a statement.