Every morning, Syrian trucks roll up to a car park near a Turkish border crossing, ready to take a consignment of UN aid back inside the war-torn country.

But so far the cargo cannot reach the area where it is needed most -- the besieged rebel-held areas of eastern Aleppo where tens of thousands of people are desperately short of food.

The United Nations has been running the operation for the past two and a half years, sending in more than 9,000 trucks to northern Syria through two UN hubs in Turkey.

At one Turkish staging post, just north of the Cilvegozu crossing that lies barely 40 kilometres (25 miles) from Aleppo, the aid is loaded onto empty Syrian trucks, their number plates covered with paper.

Turkish driver Ibrahim said they had a duty to deliver the goods to the many Syrians suffering after five and a half years of brutal civil war.

"Last year, there was snow and ice and we came without any hesitation. We are here warm, and they are in the cold," he told AFP.

"We made sacrifices, it is risky and dangerous but we do it. They are Muslims, we are also Muslims, we are brothers."

Ramesh Rajasingham, the UN's deputy regional humanitarian coordinator for the Syria crisis, said this is "the safest and quickest way" to reach out to people in northern Syria.

Rajasingham said eastern Aleppo was in a "dire situation," and that the United Nations was pushing all key players to allow the shipments through.

"In terms of east Aleppo, we just need the green light from the people who control the roads going in because, as you know, the east of Aleppo is besieged."

He said Syrian President Bashar al-Assad's ally Russia has given verbal support, and rebel groups have given written confirmation they will provide guarantees of security.

After receiving the final formal confirmation from Russia and the Syrian government, "we will move very rapidly".

With the Damascus regime pressing on with its blistering offensive, around 250,000 civilians besieged for months in eastern Aleppo have faced serious food and fuel shortages.

No aid has entered the city's devastated eastern neighbourhoods since government troops surrounded them in July.

And in a major blow Monday, rebels lost all of the northern districts in their strongholds as the army made significant advances in its offensive to recapture the entire city.

'Thorough screening process'

The UN Security Council adopted a resolution in July 2014 to allow the crossing of humanitarian assistance after difficulties faced by UN agencies in accessing affected populations in Syria.

UN agencies, in cooperation with international and local charities, provide assistance to Syrians in selected districts in areas accessible from Turkey including Idlib, Hama and Latakia, as well as parts of Aleppo province not affected by the fighting in the city.

UN officials say one million people live in 20 besieged areas in Syria that are militarily encircled with no access for humanitarian assistance, and another four million live in areas with no predictable or regular access.

The aid operation begins well before dawn in Cilvegozu, where Turkish trucks chartered by UN agencies wait to hand over their cargo.

Every vehicle is carefully screened to ensure it does not carry unsolicited items. Turkish customs authorities check the driver's paperwork and the truck is X-rayed.

"The process starts very early in the day," Jean-Luc Tonglet, officer-in-charge at the UN Monitoring Mechanism, told AFP. "The screening process is rather thorough.

"It is a process that takes 15 minutes for every single truck. If you have a convoy of 20 trucks it takes 2.5 hours."

Then starts the loading process.

Manual labourers carry sacks and boxes to Syrian trucks as UN monitors look on, to make sure for example that no people are being smuggled across.

Finally the trucks are on their way, eventually -- hopefully -- providing some relief to those in need.