The UN has called on the Syrian government to "immediately" allow life-saving aid into eastern Aleppo, where about 300,000 people are living under siege.
Speaking in Geneva, Switzerland, on Thursday, Staffan de Mistura, the UN special envoy, said there were 20 aid lorries ready to cross the border from Turkey into Syria, and make it to Aleppo city where a cessation of hostilities was holding "by and large".
"The Syrian government promised permits for UN aid convoys before the ceasefire ... they have not been received," de Mistura said. "This is something that is required to happen immediately."
Jan Egeland, secretary-general of the Norwegian Refugee Council and special adviser to the UN envoy, decried the ineffectiveness of all parties in taking advantage of the truce, saying that aid could reach Aleppo by Friday.
"Can grown men please stop putting bureaucratic roadblocks in place to stop aid workers doing their jobs to help civilians - wounded women and children?"
Al Jazeera's Stefanie Dekker, reporting from Gaziantep on the Turkish side of the Syria-Turkey border, said while progress was being made, it could take several days for aid to reach the city.
"One of the key issues is access, and one of the main arteries into Aleppo is Castello Road. It needs to become a demilitarised zone before any of that can happen," she said.
"Syrian government troops and opposition forces would need to withdraw up to 3km from the road with their heavy weapons. The deal is that both want to do this at the same time because there is no trust."
As the only supply route into the embattled part of Syria's largest city, Castello Road plays a crucial role in ensuring that residents of Aleppo, who are largely dependent on outside aid, can receive food, medical items and other essentials.
Twenty lorries loaded with much-needed food and other aid have been awaiting clearance at the Turkish border since Wednesday for the journey to Aleppo.
"I've urged the Russian government to make sure that they exercise influence on the Syrian government, and also the American side to make sure that Syrian armed groups, they also fully cooperate," Ban Ki-moon, UN secretary-general, said on Wednesday.
INSIDE STORY: Could the latest deal on Syria bring peace?
Under a deal brokered by the US and Russia on Friday, the two countries are aiming for reduced violence over seven consecutive days, before they move to the next stage of coordinating military strikes against ISIL and Jabhat Fateh al-Sham, the group formerly known as al-Nusra Front.
On Wednesday, both Russia and the US spoke positively about the truce deal, with the Kremlin saying it raised hopes for a peaceful solution to the crisis.
However, prominent Syrian opposition politician George Sabra said the many violations of a previous truce had undermined confidence in the current ceasefire.
He said it was too early to talk about a resumption of peace talks that were abandoned in April.
Estimated death toll
The Syrian civil war started as a largely unarmed uprising against President Bashar al-Assad in March 2011, but quickly developed into a full-on armed conflict.
De Mistura estimated in April that more than 400,000 Syrians had been killed.
Calculating a precise death toll is impossible, partially owing to the forced disappearances of tens of thousands of Syrians whose fates remain unknown.
Almost 11 million Syrians - half the country's pre-war population - have been displaced from their homes.
Source: Al Jazeera News And Agencies