Guterres urged all warring sides to allow aid trucks to return Thursday to the main town of Douma in Eastern Ghouta to complete the delivery of humanitarian supplies "as previously agreed with the Syrian authorities," a UN statement read.

As air strikes and artillery fire continued, aid workers on Monday decided to pull out of Eastern Ghouta after offloading 32 of the 46 trucks in the convoy.

Nearly half of the food carried on the convoy could not be delivered and part of the medical and health supplies were removed from the trucks by the Syrian authorities, the UN said.

Guterres "calls on all parties to immediately allow safe and unimpeded access for further convoys to deliver critical supplies to hundreds of thousands of people in desperate need," said the statement.

His appeal came ahead of a Security Council meeting called by Britain and France to discuss the failure of a ceasefire in Syria to take hold, 10 days after the top UN body demanded the truce.

Backed by Russia, Syria's ally, the council unanimously adopted on February 24 a resolution demanding the 30-day cessation of hostilities to allow deliveries of humanitarian aid and evacuations of the sick and wounded.

Air strikes, and artillery and rocket fire on the rebel-held enclave near the capital Damascus have killed hundreds and devastated residential areas since they began on February 18.

The Syrian Observatory for Human Rights said 800 civilians -- including at least 177 children -- have been killed since Russia-backed Syrian forces launched an assault on the besieged enclave on February 18.