South Africa's state power utility Eskom is sounding out possible buyers for its mortgage business, a tender document showed on Wednesday, in what would be the first major privatisation deal under President Cyril Ramaphosa.

Ramaphosa has made revamping state-owned firms like Eskom a top priority as he seeks to reverse years of economic stagnation and mismanagement under his predecessor Jacob Zuma.

But he faces a difficult balancing act as talk of selling state assets is anathema to large sections of society including the country's powerful trade union movement.

South Africa is exposed to Eskom via over 200 billion rand ($16.7 billion) of state guarantees, a point ratings agencies regularly cite as a risk to its sovereign rating. Eskom said in a document on a government tender website that it was considering selling all of Eskom Finance Company (EFC), a subsidiary that specialises in mortgage lending to employees.

EFC has a loan book worth around 8.7 billion rand ($721 million) and roughly 16,000 customers, documents on the tender website showed. Eskom narrowly avoided a liquidity crunch early this year by securing 20 billion rand in short-term funding from banks but still faces an uncertain future because of weak power demand and falling profits.

In February ratings agency S&P Global downgraded Eskom further into "junk" status. Eskom's new management said its balance sheet was not sustainable after the firm reported a sharp slide in profits in January.

The government said as far back as 2015 that it was considering divesting some Eskom assets, but the proposal was vehemently opposed by unions like COSATU, on which Ramaphosa relies for support. Eskom spokesman Khulu Phasiwe said the utility would hold talks with unions about the plans to sell EFC.