The blast happened in Logar province, some 10 kilometers (six miles) north of the capital, Pul-e-Alam, said Din Mohammad Darwesh, a spokesman for the provincial governor.
Darwesh said it appeared insurgents, who he identified as three Afghans and six Pakistanis, set off the bomb too early.
"Unfortunately, the explosion completely destroyed a nearby civilian house in which two women and two children were killed," he said in a telephone interview. "Police and local residents retrieved the bodies."
Two days earlier, additional Afghan soldiers deployed to the capital after receiving an intelligence report about a possible attack.
Also in Kandahar, 12 Taliban escaped from the 1,200-inmate Kandahar Prison after a prison employee falsely put their names on a list of detainees who were scheduled for release, Durrani said. The men walked out of the prison and the prison employee has disappeared, he said. Two of the 12 have been recaptured, he said.
Kandahar Prison has seen spectacular escapes in the past.
Southern Afghanistan, and Kandahar in particular, has been the site of some of the heaviest fighting in the protracted Afghan conflict as Afghan and international combat troops have tried to root out a stubborn insurgency. But the Taliban have been tenacious and most Afghans say the Taliban still rule in much of the countryside.
