The United States and Russia have called for a meeting of the UN Security Council to discuss the fragile ceasefire in Syria.
The council will hold closed consultations later on Friday.
The development comes a day after Russia's Ambassador Vitaly Churkin said he hoped the Security Council would adopt a resolution endorsing the ceasefire agreement at next week's high-level General Assembly meeting, which draws leaders from around the globe.
The move comes as clashes were reported between government forces and rebels, violating the ceasefire which came into effect on Monday.
The UN said it has not received the necessary guarantees from the US and Russia for aid convoys to cross the Turkish border into Syria - nor permission from the Syrian government for trucks to get through government checkpoints to deliver badly need supplies.
UN spokesman Stephane Dujarric said the UN needs US and Russian leadership "to have the necessary impact and influence over the various parties to ensure that the trucks can roll safely" - and the Syrian government to authorise the right administrative permissions for trucks to get through its checkpoints.
She said the UN is ready and that "it's for all the parties to see that the needs of the Syrian people are great. And every day that we're unable to move is just another day of suffering for the people of Syria, especially in the hard-to-reach areas".
The Russian military has accused the Syrian opposition of using the truce to regroup and strengthen its forces.
Head of the Russian Reconciliation Centre, Lt Gen Vladimir Savchenko, said opposition units have used the ceasefire to "restore their capability and regroup their forces in the provinces of Aleppo, Hama and Homs".
Russia pledged to help ensure the ceasefire holds in Syria for another three days, but warned the US to press the rebels to end their violations to prevent the situation from "spinning out of control".
Lt Gen Viktor Poznikhir claimed the Syrian army has fully complied with the truce while the opposition units have violated it 144 times.
Earlier the Syrian army moved its heavy weapons back to a key road near the city of Aleppo after the opposition failed to withdraw theirs in sync.
The Russian military said the Syrian army withdrew its armour, artillery and other weapons north of the Castello Road early on Thursday then moved the weapons back on Friday as the opposition units had failed to pull back theirs as agreed.
Rebels are said to have fired on government positions overnight, wounding two soldiers and prompting the Syrian army to move their weapons back to the road to prevent the rebels from advancing.
Clashes were also reported between troops and insurgents as well as shelling in two areas of the Syrian capital, Damascus.
Syrian state news agency SANA said insurgents shelled government-held areas in Qaboun, wounding three people.
The Britain-based Syrian Observatory for Human Rights say the latest fighting was concentrated in the area of Jobar, next to Qaboun.