Armed groups operating in Colombia’s Amazon are tightening their grip on the region, and that’s stalling government efforts to tackle deforestation, according to a think tank report on Thursday.

The Revolutionary Armed Forces of Colombia (FARC) dissidents, known as EMC, have the ability to slow or accelerate deforestation at will, said the report by The International Crisis Group.

“This group is the most directly responsible for deforestation in the last five years,” said Rodrigo Botero, director of The Foundation for Conservation and Sustainable Development (FCDS). “More than half a million hectares have been lost in their control zones.”

More than 40 percent of Colombia is in the Amazon-an area roughly the size of Spain. Colombia has the world’s largest bird biodiversity. Fifteen percent of the Colombian Amazon has already been deforested, according to FCDS.