The most recent troop buildup comes in response to steady advances by the Taliban. Four years ago, the insurgents were active in only four provinces. Now they are active in 33 of 34, the organizations say.

“We do not support the perspective that this constitutes ‘things getting worse before they get better,’ ” said Nic Lee, director of the Afghan N.G.O. Safety Office, “but rather see it as being consistent with the five-year trend of things just getting worse.”

Despite the spread of the conflict, humanitarian organizations say they are still able to serve Afghans in much of the country. They have to be much more careful, restricting their movements and pulling back from some areas altogether.

They use Afghan workers rather than international staff members. They avoid travel by road and take greater security precautions. They have also taken to operating incognito as a matter of routine. As a result, while insurgent attacks have more than doubled since last year, attacks on N.G.O.’s have actually declined by 35 percent, Mr. Lee said.

Because of the lack of security, CARE, like many humanitarian groups, no longer uses the country’s principal highway, the Grand Trunk Road connecting Kabul, the capital, to Peshawar in Pakistan. CARE has 10 offices around the country to manage its 1,000 employees, but its own international staff members can safely visit only four or five of them, according to a spokeswoman, Jennifer Rowell.

Likewise, there is no longer an Oxfam sign on display in the entire country, although the British-based aid group finances projects in scores of villages, mostly staffed by Afghans.

“Most N.G.O.’s don’t send foreigners to most places any longer,” said Ashley Jackson, head of policy and advocacy for Oxfam in Kabul, referring to nongovernmental organizations. Like many major aid groups, Oxfam now subcontracts much of its work in the provinces to partners, usually Afghan aid groups.