BUJUMBURA, Aug 29 (Reuters) - The African Union has asked the last rebel group fighting the Burundian government to rejoin a truce monitoring team and help salvage the peace process by the end of the year, the chief mediator said on Wednesday.

Senior members of the Hutu Forces for National Liberation (FNL) quit the team last month and disappeared from the capital Bujumbura, igniting fears that they could have returned to the bush to restart a decade-long rebellion.

"We have instructions from the African Union to complete our mission by Dec. 31 this year, which means that the FNL and everybody else has to go back to the joint ceasefire monitoring team," South African mediator Charles Nqakula told journalists.

The team, comprising FNL members, government officials and South African mediators, was set up after the FNL agreed to a peace deal in September ending more than 10 years of ethnic conflict that killed 300,000 people.

"We are ready to respect that deadline, we ask the FNL to do the same," Burundi's President Pierre Nkurunziza said after meeting with Nqakula.

The rebels, whose insurgency is seen as the final barrier to lasting peace in the coffee growing nation of 7 million, have said they are ready to return to the truce team once their security is guaranteed.

"We had security around them all the time, we were giving them maximum security 24 hours and we will continue to give them that protection, because it is part of our mandate to do that," Nqakula said.

He said a regional summit would be held soon to speed up implementation of the peace deal.

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