South Africa's cabinet said the impasse was creating a humanitarian crisis.
The current outbreak of cholera was a clear indication that Zimbabweans were becoming "victims of their leaders' lack of political will", it said.
The US ambassador has reportedly said that 294 people have died from the cholera outbreak.
The South African cabinet's strongly worded statement said:
"No amount of political disagreement can ever justify the suffering that ordinary Zimbabweans are being subjected to at the moment."
Correspondents say this is the first indication that South Africa is taking a tougher stance on Zimbabwe.