Efforts to bring aid to Syrians amid a nationwide ceasefire stalled for a second day today, 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 ceasefire agreement took effect on Tuesday and calls for full humanitarian access to besieged populations. The truce has largely held, but lifesaving aid remained stuck on the Turkish-Syrian border, UN officials said.

Syria's Government has not authorised a UN convoy carrying emergency food. UN 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 UN Office for the Coordination of Humanitarian Affairs.


"Carrying out a humanitarian operation like this is particularly complex. 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 ceasefire 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.

UN aid delivery is "lagging behind for a number of technical reasons," a senior US administration official said in a briefing with reporters.

"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 the ambiguity of the opposition's position on aid and the ceasefire.

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 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 Turkish-Syrian 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 gruelling 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.

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 defence volunteer, said. "The people cannot endure much longer without supplies."