The first aid convoy enters Eastern Ghouta in Syria after weeks of waiting to get in.
The ICRC say the 46-truck convoy carrying medical supplies and food is now racing against time.
The rebel-held district has been repeatedly bombed by forces loyal to the Syrian president and aid had been blocked from entering.
U.N Official, Ali Al-Za'tari, said: "We are not bound by the five hours, we said that it's an impossible duration for any humanitarian convoy. We organised and negotiated twelve to sixteen hours. This convoy, to offload it, it will take many hours so we may be out from Douma well after nightfall."
The World Health Organisation said government authorities removed medical material from the convoy including surgical kits, insulin and dialysis equipment to prevent it from getting in.
In the past two weeks over 700 people are believed to have been killed in the area.
The bombardment led to the UN announcing that: "This collective punishment of civilians is simply unacceptable."
The UN Security Council ordered a ceasefire that had impact on the number of casualties with both residents and aid groups reporting continued air raids.
The Syrian Observatory said on Sunday that President Assad's forces now control one quarter of the area.