Reports suggest some prisoners are starving to death
The International Committee of the Red Cross says it has begun distributing food and other supplies to thousands of Zimbabwean prison inmates.
It did not comment on conditions, but previous reports have depicted ill, emaciated detainees living in squalor.
The ICRC said food shortages in prisons were closely linked to the economic crisis in the country as a whole.
It said it was working closely with authorities to improve the situation for "the most vulnerable detainees".
Despite the ICRC's policy of not commenting on conditions in the prisons it visits, the BBC's Imogen Foulkes in Geneva says the statement is in itself an indication of how bad things are.
The ICRC says it has begun feeding 6,300 prisoners, setting up therapeutic feeding programmes - a sign of severe malnutrition.
A month ago, a secretly filmed South African TV documentary - called Hell Hole - exposed the appalling conditions inside Zimbabwean jails.
It showed sick and healthy prisoners living side by side in unhygienic and overcrowded cells.
Amid high death rates, makeshift mortuaries had been set up in prison grounds, housing prisoners' bodies crawling with maggots.
The appalling conditions were confirmed by Roy Bennett, a leading politician with Zimbabwe's erstwhile opposition MDC party - which now shares power with President Robert Mugabe's Zanu-PF.
He was imprisoned for several weeks on charges including terrorism and banditry.
After being freed on bail in March, he said his time in jail had been a "harrowing experience" which "I don't wish on my worst enemy".
"There are people there who look worse than the photographs of prisoners in [Nazi concentration camps] Dachau and Auschwitz," he said.
At one prison, Chikurubi, at least 700 of the 1,300 inmates died last year, Zimbabwe weekly The Standard reported in May.
Three days ago, the South Africa-based ZimOnline website carried an interview with Zimbabwe's Justice Minister Patrick Chinamasa.
He said his ministry had received only a fraction of the budget it had been promised for the year - $327,000 (£200,000) of $17.7m.
He said the money was going towards food and basic provisions for prisoners, but did not meet even those needs.
Mr Chinamasa said in the face of the prolonged budgetary freeze, his ministry had resorted to appealing for private donations.
Red Cross goals
The ICRC says that by the end of the year it expects to be feeding 10,000 prisoners - more than half the official figure of Zimbabwe's prison population, though the real figure is thought to be much higher.
In addition to food, the ICRC said it was providing prisoners with basic provisions such as blankets and soap.
It also plans to renovate prison kitchens and water systems in a bid to prevent the spread of diseases such as cholera.
It said it was working with the Zimbabwean authorities to try to ensure the improvements are maintained - something our correspondent says will not be easy.