The UN representative attributed the spike to heightened insecurity in parts of Aleppo, Raqqa and Hasakeh that has constrained access to those regions.
O'Brien told the Security Council the United Nations hopes to deliver aid convoys to 1.2 million civilians in 35 priority zones in July, but reiterated that Syrian authorities have hindered those efforts by limiting access and obstructing deliveries.
"The realities of the ongoing conflict and the continued interference and intransigence of the parties to the conflict present serious challenges to our ability" to deliver aid, O'Brien said.
He urged the 15 Security Council members to back UN humanitarian access requests to ensure full approval "without any conditions."
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Since January, UN aid convoys have reached 334,150 of the estimated 590,000 besieged Syrians -- the majority entrapped by government forces.
O'Brien said those deliveries mark progress but "it is but a trickle compared to the level of protection concerns, needs and suffering in besieged and hard-to-reach areas."
It is "vital that the stuttering momentum sustained on humanitarian access over the past few months continues and improves, and is significantly expanded into the second half of this year," O'Brien emphasized.
He said the UN hopes to have access to all 18 areas that the world body designates as under siege. If approved, he added that the UN and its partners would require an increased cash flow to fulfill humanitarian needs.
O'Brien also condemned attacks on Syrian medical facilities and said the Islamic State group has committed genocide and war crimes against the Yazidi people.
"There is something fundamentally wrong in a world where attacks on hospitals and schools, on mosques and public markets, on ethnic, religious and confessional groups, have become so commonplace that they cease to incite any reaction."