The governor of North Kivu province said the attackers had used heavy weapons in the raid on the jail in Beni.
At least eight of the dead are prison guards, Julien Paluku added. A curfew has been declared.
The identity of the attackers is not yet clear.
Local activist Teddy Kataliko said many self-defence militias, known as Mai-Mai groups, operated around Beni.
DR Congo has been in crisis since President Joseph Kabila refused to step down after his term ended last year.
The incident in Beni is the latest in a series of jailbreaks in the country.
Last month about 4,000 inmates escaped from a high security prison in the capital Kinshasa following an attack blamed on a separatist sect.