The eastern part of the city is held by opposition forces, but has been encircled and under siege by government forces for weeks, effectively trapping some 250,000 people.
"China has been actively supporting the mitigation of humanitarian situation in Syria", he said, noting that China has provided humanitarian aid including food and supplies for Syria as well as other countries in the region through multiple channels.
The EU's foreign affairs chief, Federica Mogherini, has condemned the latest upsurge in the civil war, saying: "The tragic images of children from Aleppo in Syria sadly show all too clearly the real human suffering of millions of people around the world caught up in humanitarian crises".
Russia, which backs Syrian forces with airpower, said on Thursday it supported the truce.
Humanitarian access to hundreds of thousands of Syrians in other combat zones has been blocked by fighting, security concerns and the Syrian bureaucracy, Mr. O'Brien said, despite an global agreement reached in May to permit truck convoy deliveries. "Once we have the green light, we can start to move assistance within 48 to 72 hours", he added.
Last week, Russian Federation announced its support for the United Nations -proposed 48-hour weekly humanitarian pauses in Aleppo.
O'Brien said no aid had been delivered in August to almost one million people in besieged and hard-to-reach areas because of fighting and Syrian government bureaucracy.
"I'm angry, I'm very angry", O'Brien said.
A crackdown by Syrian President Bashar al-Assad on pro-democracy protesters five years ago sparked a civil war, and Islamic State militants have used the chaos to seize territory in Syria and Iraq.
Omran, his siblings and parents were all plucked from the rubble following Wednesday's bombing on the Qaterji neighbourhood in rebel-held east Aleppo.
The UN submitted to Syrian authorities a request to reach almost 1 million residents in August, but received approval for less than half of the requested locations, and not a single aid convoy has moved in August, O'Brien told the Council.
In video footage Omran is seen quietly staring into space in an ambulance, before raising his arms to touch his bloodied forehead, then looking at his hand and wiping it on the orange seat.
"The perpetrator must be elsewhere", Ja'afari said.