Zimbabwe is in "meltdown" says United Nations humanitarian chief Jan Egeland following a visit to the country.
He also said President Robert Mugabe's rejection of tents for hundreds of thousands of people evicted and made homeless this year is "puzzling".
Some 700,000 people lost their jobs or homes in a government demolition programme, an earlier UN report says.
"This disastrous eviction campaign was the worst possible thing, at the worst possible time," Mr Egeland said.
The government disputes the 700,000 UN figure and says it carried out slum clearances to reduce crime and overcrowding.
"The situation is very serious in Zimbabwe when life expectancy goes from more than 60 years to just over 30 years in a 15-year span - it's a meltdown, it's not just a crisis, it's a meltdown," Mr Egeland told the BBC in Johannesburg, immediately after his four-day trip to Zimbabwe.
He pointed to "the Aids pandemic, the food insecurity, the total collapse in social services".
Mr Egeland, the UN under secretary for humanitarian affairs, said donors had an obligation to help despite disagreements with the government - of which the offer of tents was the most notable.
"If they [tents] are good enough for people in Europe and the United States who have lost their houses, why are they not good enough for Zimbabwe?" he said.
Mr Mugabe's spokesman said Zimbabweans were "not tent people" and they wanted the UN to build permanent homes.
Life expectancy 30 years
3m expecting food aid
20% adult HIV prevalence
3,000 Aids deaths each week
500,000 left homeless this year
200,000 lost livelihoods
Inflation has reached 400%
Crisis compounded by drought
Mr Egeland said the government's rationale for the eviction campaign was deeply flawed.
"The eviction campaign seems to me wholly irrational in all of its aspects - you lowered the standard of living rather than increasing it."
Mr Mugabe last week agreed to let the UN provide food aid to some three million people over the next year.
"The humanitarian situation in Zimbabwe is extremely serious and it is deteriorating," Mr Egeland said.
After "frank" talks with Mr Mugabe on Tuesday, Mr Egeland said they had agreed that the international community should do more to meet humanitarian needs in Zimbabwe.
"Our message to the government was to help us, to help you, to help your people."
And when asked why donors should fund the $276m being requested to save lives in Zimbabwe, Mr Egeland said "it is in no way punishing the government, to not help women and children in great need".
Mr Egeland spent Monday meeting people living in camps and said some of them were living in inadequate conditions - much worse than before.
When questioned on whether UN staff on the ground were negligent by failing to help Zimbabweans by seeking to avoid confrontation, he said he had raised the issue of criminal behaviour with Mr Mugabe.
"It's a criminal act to bulldoze someone's home who owned their land - there should be prosecutions."