South Africa has disclosed that it turned down a request from Zimbabwe for an emergency loan of $1.2bn (£932m) in December.
Treasury spokesman Jabulani Sikhakhane said South Africa did not have “that kind of money”.
Zimbabwe’s government had hoped to use the cash injection to stabilise the ailing economy and resolve fuel shortages in the country.
Soon after the request was rejected, Zimbabwe’s President Emmerson Mnangagwa was forced to announce a steep increase in the price of fuel – a move which has caused angry protests.
Police have violently repressed the protests that broke out mainly in the capital, Harare, and the southwestern city of Bulawayo, with some reports that they were conducting door-to-door searches and using of live ammunition.
Human rights groups say at least a dozen people have been killed, but there has been no official confirmation.
The UN called on the government to halt the “excessive use of force” by security forces.
President Mnangagwa tweeted that he was abandoning plans to attend the Davos economic summit and return home to deal with the crisis.