In June, relief workers were only able to reach five per cent of people in Syria's besieged areas, he said.
In some cases, O'Brien said, Syrian government officials removed essential items like blankets and medicine from UN convoys.
"I am shocked and outraged," O'Brien said at the UN headquarters in New York.
About 220,000 people have been killed in Syria since the start of the civil war in spring 2011 and 12.2 million people need help, including more than five million children, the United Nations says.
About 7.6 million are internally displaced and more than four million have fled Syria. A Syrian government crackdown on a pro-democracy movement in 2011 led to an armed uprising, and since then, ISIS militants have taken advantage of the chaos to declare a caliphate in swathes of territory they have seized in Syria.
"A political solution is more urgent than ever to end this futile and hopeless struggle," O'Brien said. "We must show the people of Syria that the world has not forgotten them or their country."