The Syrian regime has been accused of not allowing 20 United Nations aid trucks into eastern Aleppo, one of the key terms of the ceasefire that started on Sunday (12 September).
UN special envoy Staffan de Mistura said that the trucks are ready to move and that the Syrian government was in breach of the agreement brokered by the US and Russia, which included allowing the vehicles through simply by notifying the government in Damascus.
"We need to do more homework," de Mistura said although he did point out that there had been a drop in violence in the first 24 hours of the ceasefire.
De Mistura said eastern Aleppo was different from the rest of Syria where formal letters of authorisation were needed before aid can be delivered.
The Syrian government has warned it would not allow Turkish humanitarian aid into the besieged city of Aleppo without its permission.
A senior US official said Washington was doing all it could to ensure the UN convoy safe access.
"We have spent much of today pressing the Russians and, through the Russians, pressing the regime. The UN wanted to make sure the trucks go through unhindered by the regime and unthreatened by the opposition. And we hope to get that done today," he said.
"If an opposition group decides it doesn't want to part of the cessation and wants to carry out attacks on the regime, then they take themselves out of the cessation of hostilities," he said, according to The Guardian.
If the Geneva-brokered ceasefire holds for seven days, the US will join forces with Russia to target Islamist extremists Islamic State and the Jabhat Fateh al-Sham terrorist group, formerly known as al-Nusra Front.
The Syrian Observatory for Human Rights told Reuters it had not received a single report of combatants or civilians killed by fighting in any areas covered by the truce.