The accident took place in the Diboko area in the central Kasai province on Friday.
Those killed were "unauthorized diggers who entered an area where mining is forbidden", the province's vice-governor Gaston Nkole said.
Despite the mining ban, which the authorities put in place for safety reasons, informal miners have been coming to the site at night in search of diamonds.
The vice-governor said the latest toll, received on Saturday night, put the number of dead at six.
An investigation was underway, he added.
Mining accidents are common in the DRC, where thousands of ill-equipped informal diggers search for diamonds, gold and precious minerals.