"Until just before the capture of Corporal [Gilad] Shalit, it was a dangerous humanitarian situation and there was a significant deterioration across the board," Alvaro de Soto said in an interview with AFP on Tuesday.
Israel sent ground forces into Gaza and knocked out a power station on June 28 on the first of seven consecutive nights of air strikes over the territory.
De Soto said there was a risk of waterborne diseases without proper water distribution, sanitation and sewerage system, and he urged Israel to restore fuel supplies and "co-operate energetically" to get the transformers replaced.
"We don't want to suggest that this is like Darfur ... It's a dangerous situation, it's a situation that could get very serious over time," he said.
De Soto also said the United Nations "took exception" to Israel's arrest of a third of the Hamas-led Palestinian government and the bombings of the offices of Ismail Haniya, the prime minister, in Gaza last week.
"Palestinian institutions which have functioned and we have relied on very heavily in order to provide for the basic needs of the Palestinian people need to be preserved if there is going to be a two-state solution," he said.