Billions of dollars in fines owed by South African companies MTN and Standard Bank to Nigerian regulators could increase the risk to South Africa's financial system, the South African Reserve Bank (SARB) said on Wednesday.

The Nigerian central bank in August accused telecoms firm MTN and its lender Stanbic, a Standard Bank subsidiary, of illegally sending $8.1-billion abroad.

The Nigerian government has also demanded $2-billion in related taxes from MTN. The South African mobile operator makes about a third of its annual core profit in the West African country.

The South African Reserve Bank (SARB) said in its semi-annual review of financial stability that the repatriation claim and the $2-billion underpayment in tax amounted to approximately 100% of MTN's market capitalisation.

"Any potential impact on the South African financial system arising from this event will depend on the eventual resolution of the matters raised and MTN Group's ability to continue meeting its debt obligations," the bank said.

"A potential worst-case scenario would be for the MTN Group to disinvest from Nigeria," the central bank added.

In October a Nigerian judge set a hearing for the dispute for Dec. 4.

Some analysts have cited the concentrated foreign currency funding risk posed by MTN and a clutch of other Johannesburg Stock Exchange-listed firms as a threat to the currency and the central bank's foreign exchange reserves.

The central bank said that if there were a spillover from the market turmoil linked to external debt and the foreign currency funding crunch that hit Turkey and Argentina earlier this year, South Africa could suffer sharp capital outflows and exchange rate depreciation.

The rand is already down around 15 percent against the dollar this year, while portfolio flows slumped dramatically in the first half as a local recession exacerbated the global flight from emerging markets.