SAWRAN, Syria — The United States and other international donors are spending hundreds of millions of dollars on humanitarian aid for Syrians affected by the civil war.

But here in the rebel-controlled north, where the deprivation is most acute, that money has bought mostly resentment. The majority of aid is going to territory controlled by President Bashar Assad; the small amount reaching opposition-held areas is all but invisible.

“Aid is a weapon,” said Omar Baylasani, a rebel commander from Idlib, speaking during a visit to a Turkish border town. “Food supply is the winning card in the hands of the regime.”

The biggest obstacle blocking aid from rebel-held areas is the U.N. requirement that its relief agencies follow Assad’s rules — which limit access to opposition territory — as long as the international assembly recognizes his government. The U.N. agencies are the main conduit for international aid, including most of the $385 million that Washington has directed to the cause in 2012 and 2013.

That means while internally displaced Syrians living in government-controlled areas are cared for in U.N.-run camps, with standard shelter and basic utilities, the many who have fled into opposition territory are plagued by shortages of food, fuel, blankets and medicine.

The lack of foreign aid “is a catastrophe,” said Saed Bakur Abu Yahia, the clinic’s director. “We get nothing,” he said, bundled in a winter jacket and rubbing his hands for warmth as he sat in his office.

The United States has done more than any other country to circumvent the United Nations, but its efforts remain unknown to most Syrians.

Washington is funneling about $60 million — about $10 million in 2012 and about $50 million in 2013 — through independent nonprofit groups to deliver flour, food baskets, blankets and medicine to the most stable opposition-controlled territory. One group said it reached most of Aleppo province but not yet Idlib.

The nonprofit groups insist on keeping their work and their U.S. donors a secret to protect staff members still working in Damascus under Assad.

“Our humanitarian assistance, $385 million to date, is making a difference,” said one U.S. diplomat involved in Syria policy, speaking anonymously. “But we can’t talk about it.”

U.N. officials have acknowledged the problem, but they say they have few alternatives while the United Nations recognizes the government, which is unlikely to change as long as Russia supports Assad.

Among other obstructions, Assad has blocked any U.N. agencies from the shortest and safest route into rebel territory, across the opposition-controlled border with Turkey.

To get around that ban, at least three U.N. relief convoys have crossed the battle lines to reach parts of the north, Laerke said, but it is a dangerous trip.

Eight U.N. aid workers have been killed during the Syrian conflict, he said.