The attack occurred about a quarter mile from the governor's house in Tirin Kot town, Uruzgan province, said Abdullah Khan, the governor's spokesman.
U.S. Ambassador Ronald E. Neumann and other American officials inside the building at the time were unhurt, said U.S. Embassy spokesman Lou Fintor.
Ten Afghans standing outside the building were killed, said Abdul Aziz, the deputy provincial governor. He said 50 others were wounded, including Uruzgan's deputy police chief, and 15 were in critical condition.
A purported Taliban spokesman, Qari Mohammed Yousaf, called The Associated Press shortly after the blast to claim responsibility, saying the bomber had intended to attack the U.S. officials, but had detonated the explosives prematurely near a group of policemen.
He said the assailant was an Afghan and had hidden a land mine under his clothes.
Yousaf said the Taliban were aware that some "high-ranking Americans" were planning to visit the town but did not know who they would be.
Yousaf has claimed responsibility for previous attacks in Afghanistan, but his exact ties to the Taliban leadership are unclear.
The past year has been Afghanistan's bloodiest since U.S.-led forces ousted the Taliban regime in late 2001 for harboring al Qaeda chief Osama bin Laden. About 1,600 people have died in the violence as the Taliban and al Qaeda militants have stepped up attacks, including a spate of suicide bombings in recent months.