(CNN) A s Syria's ceasefire largely holds , residents trapped in Aleppo eagerly await crucial aid supplies after months cut off from the rest of the world.

But one main highway stands in the way of delivering aid to rebel-held eastern Aleppo: Castello Road.

All eyes are on the long, dusty route after the ceasefire negotiated between Russia and the United States was expected to include delivery of much-needed humanitarian aid to Aleppo. So far it has not.

The start of Aleppo's Castello Road, controlled by the Syrian regime forces.

As truckloads of food and medical supplies await, here's why Castello Road is a big factor in the ceasefire.

Where is Castello Road?

The highway nicknamed "Death Road" cuts through Aleppo, and is considered the only route into the eastern part of the city.

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Aleppo has been under constant bombardment in recent weeks, and getting food and medical supplies to besieged areas is a matter of life and death.

"This arrangement ... requires forces from both sides to pull back from Castello Road," US Secretary of State John Kerry said last week.

"What this pullback will do is create a demilitarized zone around it, permitting as quickly as possible the resumption of humanitarian and civilian traffic along that road."

Why is it nicknamed the 'Death Road'?

Wrecked cars and trucks line the road, and so do rows of empty, bombed-out buildings. Castello Road is a long way from a regular commute. "The road smelled of rotten flesh, burnt metal, there were plumes of smoke from ordnance that had fallen previously," said Dr. Samer Attar, who used the highway to go to Aleppo in July. With every minute, it felt like death was just around the corner.

"The driver was really fast and at every moment you felt like you would get hit by a bomb or a missile or bullet," he says.

Such scenes explain why aid agencies are staying away.

What's the connection to the ceasefire?

One word: accessibility.

No access to Castello Road means no aid to areas badly hit in Aleppo. Forces loyal to the Syrian government took over the road in July, and opposition fighters' attempts to retake it have failed.

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The ceasefire deal negotiated by the US and Russia stipulated that pro-government forces allow safe access by vacating the contested highway. The road has "special status" under the agreement, said the UN's special envoy for Syria, Staffan de Mistura.

Why not use the road if ceasefire's in place?

It's complicated.

Earlier this year, yet another ceasefire fell apart in Syria, so there's a lot more diplomatic dance and caution involved this time.

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Aid agencies have said they need reassurances from both government and opposition groups to use the road to deliver aid.

De Mistura said the two brokers of the deal, Russia and the US, are expected to produce a plan for disengagement from the road, and "are working hard to achieve that."

He would not say how much of the road is expected to be opened to allow unimpeded movement of UN convoys, but that "it urgently needed to take place."

A child carries a canister filled with water near Aleppo's Castello Road.

Once an agreement is reached, there would be "special checkpoints" set up for the aid convoys, he said.

Russian officials have said the Syrian military is ready to pull out of the highway to allow safe passage of humanitarian aid into the city, Russian state news agency Tass reported.

Syria has not confirmed or reported this news, leaving aid agencies unsure whether to access the road. Russian forces have joined their Syrian counterparts in manning the route, making the issue even more complicated.

Castello road, West #Aleppo under Russian Army control today, vehicles searched as they pass checkpoints @MmaGreen pic.twitter.com/NzcGoxMjWw — Syria Today (@todayinsyria) September 13, 2016

What's next?

The UN says Syria has to issue a letter authorizing aid delivery to Aleppo.

Russian officer caught in crossfire on Castello road, Aleppo https://t.co/Ddu9mlIfaR pic.twitter.com/8RQLrGixx2 — Ruptly (@Ruptly) September 14, 2016

Until then, aid is stuck at the Turkish border because the Syrian government has not guaranteed safe passage.

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Aid convoys are positioned at the border town of Cilvegozu, poised to enter the country and deliver food and medical aid to rebel-controlled eastern Aleppo, where the United Nations says between 250,000 and 275,000 people have been cut off from assistance since early July.

The trucks are carrying enough to feed 40,000 people for an entire month.