The recent escalation Syria's brutal conflict has complicated the already difficult task of delivering aid to the millions in need across the country, the United Nations humanitarian chief said Friday.
The fighting has stepped up since Russia late last month began an aerial campaign in the country, where a US-led coalition has been carrying out air strikes for more than a year.
"We are finding that the increased amount of aerial bombardment, from whichever source, is putting the supply routes at more risk," Stephen O'Brien told AFP in an interview.
This, he said "has meant that we have been unable to have as many convoys moving to get the supplies to the people in need."
Syria's bloody conflict, which erupted in March 2011, has left more than 250,000 people dead and forced more than half the country's population to flee their homes, including more than four million who have fled the country as refugees.
Over 12 million people in Syria are in desperate need of humanitarian aid.
O'Brien said the UN was tirelessly working to seek safe routes to bring aid to those in need.
"We are always prepared and willing to get convoys moving wherever we can get corridors, or safe-passage or a clarity that the convoys can get through," he said.
He said an aid convoy could soon make it to Zabadani, the last rebel stronghold along Syria's border with Lebanon, after a six-month truce was agreed there in September.
"As I understand it, particularly towards Zabadani and the surrounding villages and towns, there is the opportunity to get those convoys moving," he said, hinting an aid convoy might make it through Friday.
He stressed though that the situation remained blurry, and that "of course the convoys don't set off unless we have received assurances that their passage is intended to be safe."
The ceasefire is supposed to include the evacuation of civilians and rebel fighters out of Zabadani in exchange for the safe passage of civilians from the Shiite Muslim villages of Fuaa and Kafraya in the northwestern province of Idlib.
O'Brien said he did not have information about pending evacuations.
The Kremlin has said the Russian bombing campaign is aimed to help President Bashar al-Assad's beleaguered forces fight the jihadist group Islamic State.
On Friday, Moscow claimed it had hit more than 380 "Islamic State targets."
But rebels and their international backers say the intervention is intended to bolster Assad and has targeted moderate and Islamist opposition forces rather than jihadists.
Russia's entry into the conflict has raised fears of a potential confrontation with the US-led coalition.