Jordan-Syria border: Refugee children apathy
"The living conditions of the people stranded at the berm are dire," O'Brien told a press conference after visiting makeshift camps for tens of thousands of Syrians who have fled the fighting in their country.
Jordan has since June 21 blocked the passage of aid to the refugees and their entry after a suicide bombing claimed by the Islamic State jihadist group killed seven of its soldiers in the desert area.
"There is no access to basic resources such as food and water, with over 75 percent of that population estimated to be women and children," O'Brien said.
"They desperately need assistance and our humanitarian support."
However, he also called Jordan's security concerns "very legitimate", and said discussions were under way with Amman to find a way to deliver aid without endangering the kingdom's security.
O'Brien emphasised the commitment of donor countries to help Jordan ease the burden of the refugee crisis.
The country already hosts more than 600,000 Syrian refugees according to the United Nations, a figure the Jordanians put at 1.4 million.
Amman regularly complains of a lack of international aid, believing it has "reached its limits" and regularly urges the international community to share the responsibility.