Most U.S. installations in Iraq and Afghanistan are heavily guarded with rings of razor wire, huge sand-filled barriers, blast walls and security cameras. It is rare - almost unheard of - for insurgents to breach such defenses and get inside.
NATO says around 100 insurgents were killed in the battle. The local police chief was also captured by the attackers and executed, said Maj. T.G. Taylor, an American public affairs officer based in nearby Jalalabad.
Taylor said 24 Americans and 10 Afghan soldiers were wounded during the fighting, which took place in the remote district of Kamdesh, a mountainous region of Nuristan province near the Pakistan border. He said large portions of the base burned down, probably from incoming rocket and machine gun fire.
Trouble began Friday night, when insurgents comprised mostly of local Nuristani fighters began warning villagers "that something was going to go down and asked them to evacuate," Taylor told The Associated Press in a telephone interview.
It's unclear whether civilians fled, but local police units abandoned the village, he said.
Around dawn Saturday, about 200 "local fighters" launched a coordinated attack, showering a joint U.S.-Afghan army outpost with small arms, rocket-propelled grenades and mortar shells. Taylor said insurgents fired from at least three sides — including a local mosque they took over, buildings in the village, and high ground above the outpost.
Two American platoons as well as Afghan troops were in the outpost at the time. U.S. infantry platoons ordinarily number 30 to 40 soldiers.
Insurgents also attacked an observation post perched on a ridge above manned by another American platoon. Such posts are set up to keep watch over other allied positions.
Some of the insurgents had positioned themselves between the two allied posts.
"The attackers were able to breach the perimeter of one of the bases and get inside," Taylor said. "They got a foothold on the base. But coalition and Afghan national army forces consolidated their positions, retook the parts of the base the enemy was on and re-established security."
Taylor had no details of how the attackers got inside the base or how many had done so. He stressed he was not in Kamdesh at the time and his information was based on preliminary reports.
The fighting lasted about five and a half to six hours, Taylor said. Coalition reinforcements were flown in by helicopter to a nearby location around that time and traveled to the bases on foot.
Sporadic exchanges of fire continued for another few hours, Taylor said.
Coalition forces fended off the assault with "a combination of close air support and small arms fire," Taylor said. NATO officials have said the coalition used artillery and helicopter gunships.
The Afghan Defense Ministry said joint operations were launched Monday and 40 insurgents were killed by Tuesday, but Taylor and other U.S. officials said that since nightfall Saturday, there has been no contact with insurgents at either location.
Another American official said coalition forces had not pulled out of the area.
Information out of Kamdesh has been spotty because the region has no regular phone or radio contact and few roads.
Taylor said insurgents were able to get close enough to the posts to carry out the attack because it is difficult to tell the difference between fighters and the general population.
"You have local fighters who might have weapons or might not, and you have a local population that is perfectly within its rights to carry weapons," Taylor said. "How do you tell them apart?"
"When you have an insurgent who looks just like the population, like anybody, you don't know the difference until they start shooting at you," he said.