"Eleven guards and an American journalist working for the Okapi Wildlife Reserve (RFO) were abducted yesterday by the Mai-Mai Simba," Alfred Bongwalanga, administrator of Mambasa district in the province of Ituri, told AFP.
Separately, a senior official with the Congolese Institute for the Conservation of Nature (ICCN), speaking on condition of anonymity, said, "Eleven RFO park wardens and an American journalist are missing, while two Britons and five other wardens escaped, when they were attacked by the Mai-Mai Simba."
"The US Department of State has no higher priority than the protection of US citizens overseas. Due to privacy considerations, we have no further comment," the official added.
The vice governor of Ituri, Pacifique Keta, said the armed forces and "every service" had been informed of the incident.
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The names of the missing wardens and female journalist were not given.
A former Belgian colony, the DR Congo is a vast country rich in minerals and timber but wracked by decades of war and poverty.
The Mai-Mai Simba is a self-described "self-defence" militia drawn from the Nande, Hunde and Kobo communities as well as rivals from the Nyaturu, who represent ethnic Hutus.
Many of these groups were armed during the DR Congo's second war -- a conflict that ran from 1998-2003 -- to fight incursion by Rwandan or Ugandan combatants, and have never been disarmed.
The park is notably home to the okapi, an endangered zebra-like species that is a cousin of the giraffe. It is also home to the Mbutu and Efe pygmies, hunter-gatherers who are "among the last true 'forest people' on Earth," according to the Okapi Conservation Project website.