NATO Secretary-General Anders Fogh Rasmussen is in Afghanistan for his first visit as chief of the 28-nation alliance.

The former Danish prime minister says one of his key goals will be to gradually transfer security responsibility from international forces to the Afghans.

Another concern for the alliance is limiting civilian deaths. On Wednesday, villagers in southern Afghanistan said an air strike targeting a house killed four civilians, three of them children.

Villagers protested in Kandahar, chanting anti-American slogans and displaying what they said were the victims' bodies.

U.S. military officials said helicopters operating in the area early Wednesday attacked and killed four militants riding motorcycles in a field away from populated sites.  They said the militants were carrying jugs that exploded.  Officials are investigating the villagers' claims.

U.S. General Stanley McChrystal ordered U.S. and NATO troops last month to limit the use of air strikes to try to reduce such casualties.  Afghan President Hamid Karzai also has called repeatedly for a stop to aerial attacks on populated areas, saying they cause resentment among the public.

The NATO chief's unannounced visit comes a day after a series of rockets hit the Afghan capital in the largest single militant attack during preparations for the presidential elections on August 20.

On Wednesday, a roadside bomb struck a civilian vehicle in eastern Afghanistan's Nangarhar province, killing six people.  Officials say the dead include two tribal elders and four workers who were traveling to a meeting with local officials.

Taliban militants have stepped up attacks across Afghanistan ahead of the election, which they have vowed to disrupt.