Boko Haram insurgents killed at least six park rangers on Tuesday in a reprisal attack after troops backed by helicopter gunships destroyed their base in a Nigerian game reserve.
Security forces last week killed 17 militants and destroyed two Boko Haram training camps in the northeast, one in a forest and one in the Sambisa Reserve.
The officer in charge of the Sambisa Reserve, Hassan Malgwi, said gunmen believed to be Boko Haram attacked the staff camp after midnight.
"Six people were killed. Many of our staff are still missing as the camp is now deserted," he said.
Neither the military nor Boko Haram were available for comment.
Boko Haram are seen as the most serious threat to the stability of Nigeria, Africa's top energy producer, and Western governments fear it could become a base for operations of al Qaeda-linked Islamist groups in the Sahara.
Violence has persisted since a Boko Haram commander declared a ceasefire last week, raising doubts about his credibility.
President Goodluck Jonathan has highlighted links between Boko Haram and Saharan Islamists, including al Qaeda's North African affiliate, as a reason for joining efforts by French and West African forces to fight them in Mali.