The blazes began mostly as fires for hunting or clearing land that spread out of control, consuming villagers' thatched huts, aid workers said.
The Catholic charity Caritas said two men and a child had been killed by fires in the Kongolo region of Katanga province, where 996 homes had been destroyed.
The UN Office for the Coordination of Humanitarian Affairs (OCHA) said it planned to launch an appeal to bring aid to the stricken villagers.
"We have recorded 10,000 affected households in southern Maniema," with tens of thousands left homeless, said Alexis Asani Ndalimbuzi, director of natural resources in the provincial town of Kasongo.
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Idrissa Conteh, head of the OCHA office in provincial capital Kindu, said the fires had also affected shelters for people displaced by conflict -- leaving them homeless for a second time.
In conflict zones, where land disputes abound, "arson is used as a weapon of war", he added.
"We are going to alert the prime minister and the local government," said Conteh. "We will send out an SOS so we can provide humanitarian aid.