South Africa's State-run power utility Eskom has raised its latest pay rise offer to 6.7% from around 6% to three unions it is in negotiations with, a source with the National Union of Mineworkers (NUM) said on Friday.

The NUM source earlier said its members were demanding as much as 12% but would "settle" for 9% as well as a housing allowance and annual bonus, and were prepared to declare a strike on Tuesday if no settlement is reached by then.

Eskom has rejected the demand for a housing allowance and annual bonuses, the source said. The utility could not immediately be reached for comment.

The NUM is one of three labour unions locked in a wage dispute with cash-strapped Eskom, which initially refused to raise salaries this year, triggering protests and a spate of rolling black-outs in Africa's most industrialised economy.