Their guns are hidden from view, tucked away beneath neutral khakis and polo shirts. There is no sign on the door to their heavily fortified compound. When they cruise around the city, they ride in a fleet of unmarked sport-utility vehicles.

U.S. Special Forces, the vaunted soldiers who have taken the lead in the hunt for Osama bin Laden, are also involved in a more benevolent undercover operation in Afghanistan: the largest and fastest humanitarian deployment by the military since it got into the assistance business in the 1990s in such places as Somalia and Haiti.

In the past, the U.S. military has waited until the bombs stopped falling before handing out aid. This time, a force of about 150 is here even as the war continues. They are supervising road building and school construction, well digging and crop planting.

But the nature of the humanitarian mission -- and the civilian dress of soldiers in its ranks -- has angered nongovernmental humanitarian organizations. Civilian aid workers say the military's efforts endanger the effectiveness of their longer-term projects, and the near-term safety of their employees.

"It's not purely a hearts and minds operation," said Sally Austin, country director for CARE International. "They are here in civilian clothes, saying they are doing humanitarian work. But they are blurring the lines. They are putting our own efforts as humanitarians at risk."

U.N. officials and nongovernmental groups said they have protested the military's use of civilian cover to the highest levels of the Pentagon. Some aid organizations flatly refuse to work with soldiers, saying that the military's aid efforts undermine the principle of neutrality that allowed aid groups to keep working in Afghanistan even during the toughest years under the Taliban government.

To these critics, the ambiguity of the soldiers' dress also raises broader questions about the U.S. engagement in Afghanistan, a project that continues to emphasize military might over civilian reconstruction even as U.S. officials tout their interest in helping rebuild the shattered country.

Those in charge of the operation describe it as a hybrid mission that recognizes that U.S. soldiers are still fighting and dying in the name of ridding the country of al Qaeda terrorists and their allies. "We are here for a short time as a bridge between war and peace," said Brig. Gen. David Kratzer, who commands the Coalition Joint Civil Military Operations Task Force, known as Chickmotif for short.

"The war's still going on. We're soldiers. We don't try to hide that fact," Kratzer said. However, he said, civilian dress is the only way to ensure the safety of his troops. "We couldn't do what we're doing and walk around in uniforms."

Although the U.S. administration has expressed an aversion to the term "nation-building," an explicit goal of the operation is political: to shore up the U.S.-backed government of the interim leader, Hamid Karzai. "Why are we here? We only had six months to support the interim administration," Kratzer said. "We can't wait until things are absolutely safe."

Chickmotif has set up its headquarters in a tattered Kabul compound christened Taliban House in honor of its former occupants. Small teams of four to six men each are already scattered in other cities across Afghanistan, including Herat in the west, Mazar-e Sharif and Kunduz in the north, Khost in the east, Kandahar in the south and Bamian in the central part of the country. The teams are nicknamed Chicklets.

Apart from clothing, there is no mistaking the military cast of the operation. Polite young reservists with short haircuts bustle about with duty rosters. There are acronyms for everything, and conversations about planned aid projects inevitably return to the collision between the precise demands of bureaucratic forms-in-triplicate and the chaos of a country after 23 years of war.

A reservist who is an administrator at the University of Florida, Kratzer led the first wave in, arriving on Christmas Eve. He brought with him civil engineers, public health specialists, a hydraulics expert, a veterinarian, a dentist and even two military lawyers.

Kratzer's budget for the year is a modest $5 million in a country ravaged by war and drought. The U.S. soldiers on his team landed with no Afghan expertise and no plan for how to help. They have scrambled to build a to-do list.

The team has come up with a list of 130 projects they hope to start, with an initial infusion of more than $2 million in congressionally approved funds. They include a $200,000 proposal to repair the road from Kabul to Bagram air base, a major public works project that could employ hundreds of Afghans, and a $3,800 plan to rebuild a girls' school in Mazar-e Sharif that was hit by U.S. bombs.

"Money means a different thing here," Kratzer said. "We think our $2 million is comparable to maybe $20 million in the States."

But only about 30 projects have secured final approval from headquarters in Kuwait, and Kratzer has spent much of his time making the rounds of Karzai's cabinet, securing wish lists from ministries that have yet to receive any funding.

A few ground rules for the military's operation have already been established. "There are definitely things we don't want to do," Kratzer said. Livestock is one example. "If it has a face, we don't want to be part of it. If somebody wants sheep or goats, we can't help them. We don't do horses, goats or sheep."

The military also makes clear that it is not here to repair damage done by the U.S. bombing campaign, exactly the sort of humanitarian work most nongovernmental groups believe the military should be doing. Although about five of the proposed projects would fix facilities damaged by U.S. forces, "We're not the bombing damage inspectors," as Maj. Greg Jicha, leader of the teams covering southern Afghanistan, put it.

Despite the modest accomplishments so far, soldiers say the program has ambitious aims. "Everyone here is helping to add stability and credibility to the interim government," Maj. Mike Warmack said.

Such assertions don't sit well with the aid workers who see rebuilding Afghanistan as a long-term vocation and are suspicious of the military, especially as it pursues its war aims while carrying out aid work.

Anita Anastacio, who runs projects in central Afghanistan for Mercy Corps International, said the confusion about the U.S. military's intent has been a concern since the bombing campaign started in October, when the military dropped yellow cluster bombs on Afghanistan while other planes rained down yellow packages of ready-to-eat meals for the famine-struck population.

Anastacio said that when she confronted Kratzer at a meeting last month, he told her that wearing uniforms would make humanitarian work impossible. He also said that the soldiers were instructed to carry a document, translated into Dari, identifying them as members of the military. She said she told him: "This is a country where 80 percent of the people can't read and write. It's just not clear to everyone who you are."

Under the Geneva Conventions, soldiers are not required to wear uniforms at all times but are obliged to carry their weapons openly to distinguish themselves from civilians.

Afghans say they don't care whether the soldiers wear uniforms but contend the U.S. humanitarian program has been inappropriate in other ways. It is already shaping up as a clash between the expectations generated by American cash and the limits of a federally funded program with little of that money to spend.

According to several top Afghan officials, the U.S. aid is too slow in coming and modest in scope to lend credibility to Karzai's administration. "They just come and ask questions and make promises," said the agriculture minister, S. Hussain Anwari. "But we've received nothing so far; it's just words."

Anwari said he was still waiting for a single dollar to help the spring planting, which should be 90 percent finished by now. Kratzer called spring planting "a top priority."

Then there is the case of the 100 wells.

The deputy minister of rural rehabilitation and development, Mohammed Naim Nazari, told Kratzer the military should help dig 1 million wells to cope with the effects of the drought. Kratzer agreed to dig 100.

"Of course it's not enough, especially during these years of drought," Nazari said. "It's not enough, even for a small area."

By the time Kratzer came calling, Nazari said he had already met with 70 or so Westerners who wanted to help -- U.N. officials, foreign diplomats and aid groups. "The Americans came at the end," Nazari said.

"In fact, none of the money from any foreigners has come through yet," Nazari said. "People had high hopes after the Taliban that the foreigners would help us rehabilitate the country. But we are slow, we are late. Now people are feeling disappointed and frustrated."

Over a cup of coffee in Taliban House, Capt. Greg Johnson hinted at the complexities involved when the military engages in humanitarian projects. A lawyer in the 3rd Army's judge advocate corps, Johnson is responsible for ensuring compliance with U.S. law in a country where gunmen rule and courts don't exist.

He spends his days worrying about dealing in Afghanistan's all-cash economy and how to get financial references for businessmen operating out of mud offices. He combs the fine print of proposals to ensure they "satisfy a basic humanitarian need -- food, shelter, education," and "no-goes" proposals that don't pass muster. Anything that smacks of "nation-building," he said, earns a "non-concur," including a request for vehicles to run the Kabul International Airport.

And those are the simple issues.

Asked about how the military guards against child labor, which is a regular feature of life here and a major problem faced by the international aid groups, he said, "Well, a typical contract provision requires our contractors to comply with federal law as it relates to fair labor practices, and that would certainly include child labor."

But, he said, "it would be hard to police something like that."

Equally troublesome is the question of federal nondiscrimination law, which requires that contractors don't discriminate on the basis of race, sex or ethnicity. That may be a practical impossibility in Afghanistan, with its tribal rivalries among the Pashtun and Tajik, Hazara and Uzbek.

Johnson said the teams in the field are instructed to get "verbal assurance" that everyone will be welcome to benefit from the aid. In the end, he said, discrimination may be "the lesser of two evils" if the alternative is no aid at all.

Afghan soldiers and U.S. Special Forces, in background, man checkpoint 18 miles north of Khost. Special Forces are involved in the largest and fastest aid deployment by the U.S. military in years.