BUJUMBURA, Sept 9 (Reuters) - Burundi rebels refused on Sunday to rejoin a truce monitoring team they quit in July unless the South African chief mediator of talks with the government is replaced.

The team -- comprising FNL members, government officials and South African mediators -- was set up after the FNL agreed to a peace deal in September to end more than a decade of civil war that has killed 300,000 people.

The Forces for National Liberation (FNL) -- the last active rebel group in the tiny central African country -- accused Charles Nqakula of bias.

"We have informed the international community that our movement is no longer accepting Charles Nqakula as mediator because he showed that he is on the government side", said FNL spokesman Pasteur Habimana.

"Now we want Nqakula and all his team to be replaced", he told Reuters by telephone from the Tanzanian city Dar Es Salaam.

South African officials were not immediately available for comment.

Habimana accused mediators and the Burundi government of supporting an FNL splinter group which clashed this week with fighters loyal to FNL leader Agathon Rwasa.

At least 20 were killed in the fighting on Tuesday in the northern Bujumbura suburb of Buterere.

Government officials urged the international community to step up pressure on the FNL, whose insurgency is seen as the last barrier to lasting stability in a country emerging from ethnic conflict.

"If the FNL continues to refuse joining the peace process, the international community must take sanctions against FNL leaders", said the army's deputy chief, Major General Godefroid Niyombare, who is also head of the government team at the joint ceasefire monitoring team.

Rebels have twice walked out of the truce monitoring team.

The African Union has urged the government and FNL rebels to finish their talks by the end of December this year.

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