South Africa's economy remains at risk from an interest rate increase in the United States due to its still rising levels of external debt, the central bank said on Wednesday, and hours ahead of a policy decision by the Federal Reserve.

"South Africa’s high external financing requirements make the country vulnerable to the possibility of market volatility, which is associated with increases in U.S. policy rates,” the Reserve Bank said its September financial stability review.

The bank also cited the risks to the currency posed by an impending rate hike by the Fed, which concludes it’s second to last policy meeting later this evening.

It warned that an increase from near zero rates by the U.S. central bank increased the likelihood of capital outflow.

“This could raise foreign-exchange related risks and increase the prospects of capital-flow reversal as investors become wary of exposures to volatile and vulnerable currencies,” the bank said.

The rand has fallen close to 15 percent against the dollar in 2015, tumbling to an all-time low in August as investors bailed on the unit with a U.S. rate hike looking almost certain.

The currency stumbled to a month low in the wake of last Wednesday’s budget, where Finance Minister Nhlanhla Nene again cut growth forecasts and said government would have to borrow more to plug a gap in revenue hit by high public wage increases and falling tax receipts.

The SARB's Financial Stress Index, which analyses developments domestically and globally on a continuous basis, rated as “high” the possibility of increased risk aversion triggered by changing perceptions in the timing of a U.S. rate hike.