UN aid trucks carrying food for besieged civilians in Aleppo remained stuck at the Syrian border Friday (September 16th), AFP reported.

Under the truce deal negotiated by the US and Russia, which took effect Monday, fighting is to halt across the country except in areas where extremist groups are present.

The UN has described the ceasefire as a "critical window of opportunity" to deliver aid to opposition-held districts of Aleppo city, where around 250,000 civilians are under regime siege.

The UN had hoped that 40 trucks of food -- enough to feed 80,000 people for one month -- could be delivered to east Aleppo as soon as possible.

But on Friday morning, the trucks were still waiting at the border with Turkey, said David Swanson, a spokesman for the UN Office for the Co-ordination of Humanitarian Affairs (OCHA).

"The challenge we continue to face -- and this is the very sad reality -- is ensuring all parties to the conflict, and those with influence over them, are in agreement," he said.

Under the truce deal, the main route for humanitarian assistance into divided Aleppo, the Castello Road, would be demilitarised and aid convoys would enter from Turkey.

Once aid is delivered and the ceasefire holds, the US and Russia were to begin co-operating in jointly targeting extremist groups fighting in Syria.

A military source told AFP on Friday that Syria's army "has carried out its pledge and handed over a number of points to the Russian monitoring teams" but that opposition groups had not pulled back from their positions.

Meanwhile, the UN Security Council will hold an urgent meeting Friday to hear details of the US-Russian deal on Syria.

Council members will meet at 5:30 p.m. for the closed-door consultations.

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are due to attend the council talks, held on the sidelines of the UN General Assembly meeting.