KABUL (AP) — Insurgents fought their way inside an American base in Afghanistan last weekend in a rare security breach before they were driven back under heavy fire during the deadliest battle for U.S. troops in more than a year, a U.S. official said Wednesday.

The bold assault raised serious questions about the security of thinly manned outposts spread across the troubled nation’s volatile border region with Pakistan, and reflects growing insurgent resolve.

It comes as pressure is building on the Obama administration to decide a way forward in the conflict. Wednesday marked the eighth anniversary of the U.S. invasion that ousted the Taliban for harboring Osama bin Laden after the Sept. 11, 2001, attacks on Washington and New York.

Saturday’s nearly six-hour battle in mountainous Kamdesh district, near the eastern border with Pakistan, left eight American and three Afghan soldiers dead – one of the heaviest U.S. losses of life in a single battle since the war began.

NATO says around 100 insurgents were also killed.

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.

Maj. T.G. Taylor, an American public affairs officer, said it was unclear how the attackers penetrated the base or how many there were. He stressed he was not in Kamdesh at the time and his information was based on preliminary reports.

Taylor said 24 Americans and 10 Afghan soldiers were wounded during the fighting. Large portions of the base burned down, probably from incoming rocket and machine gun fire, he said.

The evening before the attack, 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 from nearby Jalalabad.

It’s unclear whether civilians fled, but local police units abandoned the village — nearly all except the police chief who was later captured and executed.

In Washington, the Pentagon said the remote outpost was slated to close as part of a consolidation of far-flung bases.

“The Taliban was trying to claim credit for driving us out of this combat outpost,” Pentagon press secretary Geoff Morrell said, but the decision was already made.

“We need to focus our resources on those areas where they can have the biggest impact on population centers,” Morrell said.

The assault began around dawn Saturday.

Some 200 fighters bombarded a joint U.S.-Afghan army outpost with small arms, rocket-propelled grenades and mortar shells. Fire came from three sides simultaneously — including a local mosque they took over, buildings in the village, and high ground above the outpost.

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.

Nuristan Gov. Jamaludin Badar said that within the first minute alone, militants unleashed 32 rockets and four artillery shells.

“They were having trouble identifying the location of the attackers,” Badar said of Afghan troops defending the bases. “They were having trouble figuring out where the fire was coming from.”

Only three American platoons were deployed at the two posts, mostly troops from Task Force Mountain Warrior of the 4th Brigade Combat Team, 4th Infantry Division, based at Fort Carson outside Colorado Springs. U.S. infantry platoons ordinarily number 30 to 40 soldiers.

Badar said several Afghan army checkpoints in Kamdesh were overrun.

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.

But the worst of the battle came when 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.”

Close-quarters combat would have been likely at such a time – a rarity in both the Afghan and Iraq wars for U.S. troops.

Fighting dragged on for about five and a half to six hours until coalition reinforcements were flown in by helicopter to a nearby location, Taylor said. The reinforcements could not land in the area and traveled to the bases on foot.

Sporadic exchanges of fire continued for another few hours, Taylor said.

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.

The skirmish marked the heaviest U.S. loss of life in a firefight since July 2008, when nine American soldiers were killed in a raid on an outpost in Wanat in the same province.

Badar, the Nuristan governor, said Kamdesh is so isolated that local police can only be supplied by helicopter. Vehicles can barely traverse roads to the area. Phone lines and cell phone reception are hard to come by.

The top U.S. commander in Afghanistan, Gen. Stanley McChrystal, plans to shut down such isolated strongholds and focus on more heavily populated areas as part of a new strategy to protect Afghan civilians.

Taylor said plans had been in place for a while to withdraw from the Kamdesh outposts, though U.S. forces have not yet done so.

Badar, however, said “pulling out would be a mistake. We should be increasing forces, not withdrawing them.”

He said it was already nearly impossible to stop insurgents from ferrying arms, ammunition and fighters between Afghanistan and Pakistan across some 60 footpaths in Kamdesh and a neighboring district.

Any withdrawal would enable insurgents to extend their control over the region and use it as a base to train and launch attacks deeper into Afghanistan, Badar said.

Taylor said the attackers were made up mostly of local fighters from the area. Badar said the insurgents were led by a Taliban commander named Mullah Dost Mohammad who crossed the border from Pakistan four nights before the assault.

Asked how insurgents were able to get close enough to carry out the attack, Taylor cited difficulties distinguishing fighters from 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.