"O'Brien's convoy was denied passage at the final checkpoint before crossing the frontline" into Taez, said a statement by the UN's Office for the Coordination of Humanitarian Affairs (OCHA).
He was denied access to the flashpoint city "despite having received assurance of safe passage by all parties", the statement said.
OCHA did not name the party that prevented O'Brien's convoy from proceeding, but the route from the north to Taez is controlled by the rebels and their allies.
"O'Brien was extremely disappointed that humanitarian efforts to reach people in need were once again thwarted by parties to a conflict, especially at a time when millions of Yemenis are severely food insecure and face the risk of famine," it added.
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A local official told AFP earlier that O'Brien was stopped at a rebel checkpoint in Hizran, 15 kilometres (nine miles) northwest of Taez, while the government-run news agency Saba accused Huthi rebel forces of opening fire at his convoy.
The convoy travelling from the capital Sanaa had to change course because of shelling on the road to the southwestern city, said police captain Oussama Al-Charaabi, head of government security services in Taez.
The UN source said O'Brien was now slated to visit a school in the southwestern Ibb province housing internally displaced Yemenis from the Red Sea coastal town of Mokha, controlled by government loyalists since January.
Forces loyal to President Abedrabbo Mansour Hadi, backed by a Saudi-led Arab coalition, have held out in Taez where they are surrounded by the Iran-backed Huthis and their allies.
O'Brien, who is on a week-long tour of Yemen, Somalia and Kenya, warned yesterday that seven million Yemenis face "serious risk of famine" unless international donors intervene.