"Houthis are blocking supply routes and continue to obstruct the delivery of urgently needed humanitarian aid and supplies into Taiz city," said O'Brien, the under-secretary-general for humanitarian affairs.
Trucks carrying aid are blocked at checkpoints and "only very limited assistance has been allowed in," he added in a statement.
O'Brien said it was 'unacceptable' that some of the aid destined for the city's needy was being diverted away from those people.
Some 200,000 people are in need of drinking water, food, medical treatment and other life-saving assistance in Taiz, a 'deeply concerned' O'Brien said.