South Sudan troops will attack the main stronghold of rebel forces loyal to former Vice President Riek Machar if the government's offer of a ceasefire is rejected, a senior minister has said.

The government offered an olive branch to the rebels on Friday, proposing a ceasefire and saying it would release eight of 11 senior politicians - widely seen to be Machar allies - arrested over an alleged coup plot against President Salva Kiir. But Machar reacted coolly to the truce offer, saying any ceasefire would need to be properly enforced for him to take it seriously.