Leipzig, Germany  - A German woman biology student has been missing for the past five days in the central Congo after going out to observe bonobo primates at a remote site in a national park, a science institute in Leipzig, Germany said Tuesday.

Esther Carlitz, 23, went out with a male helper to try to spot the apes. Unaccountably they became separated and he returned alone to the base run by the Max Planck Evolutionary Anthropology Institute at Lui Kotal in the Salonga National Park.

The institute said staff and students were forbidden to walk alone in the forest. It was unclear why fresh-faced, bespectacled Carlitz, the daughter of a Lutheran pastor, had done so.

Professor Christophe Boesch said in Leipzig he was optimistic that if Carlitz was just lost, she would survive.

"Lack of water is not a problem there," he said.

The institute is there to learn more about the social habits of the bonobo, a chimpanzee relative.

Experienced helpers and satellite phones are to arrive at the base on Wednesday to boost the search for Carlitz.

German news reports said reaching Lui Kotal requires a 3-hour drive from the nearest town followed by a 25-kilometre hike.

The German foreign ministry said its mission in the Congolese capital Kinshasa was in touch with the Carlitz family.

Agnes Carlitz, a sister, said one of the family would travel to Congo to assess the situation. (dpa)