KABUL, Afghanistan — An insurgent raid that penetrated an American outpost in eastern Afghanistan, killing nine soldiers, has deepened doubts about the American military’s effort to contain Islamic militants and keep locals on its side.

Moving in darkness before dawn Sunday, some 200 fighters surrounded the newly built base in a remote area near the Pakistan border without being spotted by the troops inside, the provincial police chief, General Mohammad Qasim Jangalbagh, said.

He said people in the adjacent village of Wanat aided the assault. About 20 local families left their homes in anticipation of the raid, while other tribesmen stayed behind “and helped the insurgents during the fight,” Mr. Jangalbagh said.

The result was the deadliest incident for U.S. forces in Afghanistan since June 2005, when 16 American soldiers were killed as a rocket-propelled grenade shot down their helicopter.

Violence has been increasing in Afghanistan, and many people are questioning whether the Taliban-led insurgency is gaining, not losing, momentum seven years after the hard-line Islamic regime was ousted by an American-led invasion.

The coordinated assault at Wanat sent a strong signal to other insurgent groups that “America cannot resist them anymore,” said Tamim Nuristani, who was fired as provincial governor last week by President Karzai’s administration for criticizing an American airstrike that Afghan officials say killed civilians July 4 in the same area as Sunday’s attack.

Mr. Nuristani said the attackers at Wanat were a mix of Afghan- and Pakistan-based militants, some with Al Qaeda links — a sign, he said, that cooperation is growing between what had been often fractious factions fighting the Western military presence in Afghanistan.

“The [attackers] were not only from Nuristan but from other districts,” Mr. Nuristani said. “They are not only Taliban. They were [Pakistan-based] Lashkar-e-Tayyaba, Hezb-i-Islami, Taliban and those people who are dissatisfied with the [Karzai] government after these recent incidents. They all came together for this one.”

The attack — which American and NATO officials said happened in Kunar province but which Afghan officials said was in neighboring Nuristan — reinforced recent assessments by American officials that militant attacks are becoming more complex and better coordinated.

A NATO official said the attackers used houses, shops and a mosque in Wanat for cover during the hours-long battle.

The militants showered the small base — which had been established just three days earlier — with machine guns, rocket-propelled grenades and mortar shells, the official, who spoke on condition of anonymity because he wasn’t authorized to release the information, said.

Some of the militants breached the wall and got inside, killing nine American soldiers and wounding 15 others, he said.

Other American soldiers managed to drive out the attackers and called in air support. Attack helicopters swooped over the battlefield, and in hours of fighting dozens of insurgents were killed and about 40 were wounded, the NATO official said.

The official described the militant raid as “serious,” but also said it was a rarity for insurgents to get inside a base.

In Washington, the State Department spokesman ,Sean McCormack, expressed regret that American lives were lost, but argued the attack was a sign of the pressure being put on the insurgency.

“Instead of looking at it necessarily from the perspective of the Taliban or terrorists being more aggressive in coming after NATO or U.S. forces or Afghan forces, in this particular case it was an example of NATO, U.S., and Afghan forces being aggressive in combatting cross-border infiltration,” Mr. McCormack said.

The U.S.-led coalition and NATO military mission, which together have about 60,000 soldiers, have long maintained that insurgents can now operate only in small groups and have lost every battle with the militarily superior Western forces.

Yet the insurgents’ ability to assemble a large militant force to launch the attack undetected and with the apparent complicity of locals is a worrying signal for American commanders.

A Western official with detailed knowledge of the area said the raid underlined questions about the military campaign against the Taliban.

There is “overwhelming evidence that anti-coalition elements are operating effectively and that our counterinsurgency strategy is not successful … because it has not addressed the most basic need to bring security to the people and devised a means to separate the people from the enemy,” the official, who agreed to discuss the sensitive issue only if not quoted by name, said.

Nuristan has been a tough nut to crack for any central authority for centuries and was a hub of resistance against the Soviet occupation of Afghanistan in the 1980s.

Its rough terrain — mountains with forests, deep ravines and countless caves and gorges — provides a well-protected gateway from Pakistan’s lawless tribal areas where Al Qaeda and Taliban fighters find haven.

A NATO spokesman in Kabul, Mark Laity, said it was not yet clear whether militants had crossed from Pakistan to conduct the attack.

“Obviously that area of Afghanistan is close to the border with Pakistan and the Pakistan border has been relatively open,” Mr. Laity said.

The number of attacks in eastern Afghanistan have gone up 40% compared to last year, American military officials say.

“We put that down to the fact that many insurgents are able to move pretty freely across the border and obviously we need to minimize that as far as we can,” Mr. Laity said.

The Afghan government yesterday accused Pakistan’s army and its intelligence service of supporting the insurgency, saying it suspended a series of bilateral meetings planned for coming weeks. Pakistan, which formally supported the Taliban before the terrorist attacks of September 11, 2001, denies the allegations.