KABUL – The top U.S. general in Afghanistan will soon formally order U.S. and NATO forces to break away from fights with militants hiding in Afghan houses so the battles do not kill civilians, a U.S. official said Monday.

The order would be one of the strongest measures taken by a U.S. commander to protect Afghan civilians in battle. American commanders say such deaths hurt their mission because they turn average Afghans against the government and U.S. and NATO forces.

Civilian casualties are a major source of friction between Afghan President Hamid Karzai and the U.S. The U.N. says U.S., NATO and Afghan forces killed 829 civilians in the Afghan war last year.

Gen. Stanley McChrystal, who took command of international forces in Afghanistan this month, has said his measure of effectiveness will be the "number of Afghans shielded from violence," and not the number of militants killed.

McChrystal will issue orders within days saying troops may attack insurgents hiding in Afghan houses if the U.S. or NATO forces are in imminent danger and must return fire, said U.S. military spokesman Rear Adm. Greg Smith.

"But if there is a compound they're taking fire from and they can remove themselves from the area safely, without any undue danger to the forces, then that's the option they should take," Smith said. "Because in these compounds we know there are often civilians kept captive by the Taliban."

McChrystal's predecessor, Gen. David McKiernan, issued rules last fall that told commanders to set conditions "to minimize the need to resort to deadly force."

But McChrystal's orders will be more precise and have stronger language ordering forces to break off from battles, Smith said.

In the most recent civilian deaths case, a May 4-5 battle between U.S. and Afghan forces and militants in western Farah province killed dozens of civilians. A U.S. report last week said U.S. forces killed an estimated 26 civilians. However, Karzai's government says 140 were killed, while an Afghan human rights group says the number is about 100.

In the latest violence, a suicide bomber on a motorbike killed seven civilians Monday when he drove into the center of an eastern Afghan city and set off explosives.

It was unclear who the bomber was targeting when he detonated a bomb on his motorbike in front of Khost city's electric power headquarters and then explosives on his body a few minutes later, said Kuchi Naseri, a spokesman for the governor of Khost province. The Interior Ministry said seven people were killed.

There were no military or police nearby, Naseri said, but added the later blast may have been planned to hit police or officials rushing to the scene. Another 30 people in the area were wounded, he said.

In southern Kandahar province, meanwhile, another suicide bomber killed three Afghan soldiers in an attack on a convoy of troops inspecting a highway bridge for explosives. The attacker drove a car into the convoy and it exploded, said Zadi district Police Chief Niaz Mohammad Serhadi.

Serhadi said two civilians were also wounded in the blast, along with five other soldiers.

In eastern Nangarhar province, an explosion at a weapons cache killed a 6-year-old boy and wounded 20 others, police said.

It was unclear what sparked the chain reaction of explosions in caves used to store weapons and other material collected from insurgents on the outskirts of Jalalabad city, said Nangarhar province police spokesman Ghafor Khan.

"We are still investigating the incident. It is possible that the explosives ignited on their own," Khan said.

The caves where the weapons were stored were about 100 yards (meters) away from a village, and the blast shot some shells or other items into the residential area, Khan said. Two soldiers who guarded the cache were among the wounded.

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Associated Press writers Amir Shah in Kabul and Noor Khan in Kandahar contributed to this report.