Protesters in Aleppo, Syria,march to demand U.N. aid on Sept. 13 after a cease-fire took effect. (Modar Shekho/Associated Press)

Efforts to bring aid to Syrians amid a nationwide cease-fire stalled for a second day Wednesday, the United Nations and activists said, challenging a key part of a deal brokered by the United States and Russia to curb the violence and ease civilian suffering.

The cease-fire agreement took effect Monday and calls for
full humanitarian access to besieged populations. The truce
has ­largely held, but lifesaving aid remained stuck on the ­Turkey-Syria border, U.N. officials said.

Syria’s government has not authorized a U.N. convoy carrying emergency food, the agency said Wednesday. U.N. officials are worried about attacks by rebels who have rejected the deal.

“You’re talking about not only the Syrian government but also dozens of armed groups across the country — some of whom may have an agenda of their own,” said David Swanson, a spokesman for the U.N. Office for the Coordination of Humanitarian Affairs.

“Carrying out a humanitarian operation like this is particularly complex,” he said. “And there are some parties to the conflict that are not fully on board.”

The fact that the aid agencies cannot reach blockaded areas — even as the fighting subsides — speaks to the gravity of Syria’s humanitarian crisis, and to how difficult it is even for world powers to impose their will on the warring parties.

The United States has backed some anti-government rebels, while Russia last year intervened on the side of the Syrian government. The cease-fire is the second attempt by the two countries to temper fighting that has now killed nearly half a million people. A cessation of hostilities brokered in February soon collapsed, and fighting intensified, killing scores.

U.N. aid delivery is “lagging behind for a number of technical reasons,” a senior U.S. administration official said in a briefing with reporters Tuesday.

“We spent much of the day pressing not just the Russians — and, through the Russians, the regime — but also some of the opposition parties to make sure that we can get the kind of unfettered humanitarian access that our agreement with the Russians calls for,” the official said.

A commander with the Jaish al-Mujahideen rebel brigade demonstrated Wednesday the ambiguity of the opposition’s position on aid and the cease-fire.

No one on the rebel side “will restrict the flow of humanitarian aid,” said Abu Qaitaiba, who is based in Aleppo.

“Unless it is part of a humiliating deal,” he said. “Which the Syrian people will not accept.”

The United Nations said Wednesday that reaching residents in east Aleppo, a city divided and devastated by the war, is the agency’s priority. At least 275,000 people in the rebel-held east have been “almost entirely cut off from vital supplies, including food, water, medicine, electricity,” it said.

The 40-truck convoy now on the Turkey-Syria border has enough food rations to feed about 40,000 people for a month, aid workers said. But the aid has been held up by a grueling process requiring dozens of approvals from senior and local Syrian officials.

The Muslim holiday of Eid al-Adha has also slowed progress, an aid official in the region said. The official spoke on the condition of anonymity because of the sensitive nature of aid delivery in Syria.

In Aleppo, residents said they need fresh produce, fuel and medicine for hospitals and clinics that have been targeted in government attacks. The prepared rations, however, include such items as rice, bulgur wheat, salt, sugar and vegetable oil.

“We are in desperate need of medicine and so much else,” Aref al-Aref, a medic and civil defense volunteer, said Wednesday. “The people cannot endure much longer without supplies.”

At the center of efforts to bring aid to Aleppo is a violent struggle over Castello Road, which is the only way into the eastern part of the city. The Syrian government controls the highway but is expected to withdraw to allow aid agencies access.

Pro-government media reports and rebel commanders say that the Syrian army’s elite Tiger Forces unit already has pulled back from the road and that Russian marines are now manning it.

The agreement envisions Castello Road as “a demilitarized zone, where there could be greater traffic, greater humanitarian traffic, unimpeded and unthreatened,” the U.S. official said.

But since the cease-fire began, some residents of eastern Aleppo have staged demonstrations against U.N. aid coming through Castello to the city. Many there consider the United Nations, which works with the Syrian government for humanitarian access, complicit in the siege.

The aid is aimed at convincing the international community that “the U.N. is doing their best to help civilians,” Abdulkafi ­Alhamdo, a teacher and resident of east Aleppo, said of the terms of the cease-fire. “But in fact they took sides with Assad years ago.”

Mohab Abdelsalam, a rebel fighter with the Aleppo Revolutionaries, said his group has not been in touch with the United Nations regarding the aid convoy’s safety.

“There are Russian troops [on the road], and we don’t like it,” said Abdelsalam, 26. But “we will allow [the aid] because our people are suffering.”

Habib reported from Berlin. Zakaria Zakaria in Istanbul contributed to this report.