The Kamrup East Forest Division has undertaken a determined drive against encroachers in reserve forest areas in and around the city and has already demolished 250 houses so far in Fatasil-Ambari area of the city since the drive was launched on June 12, Forest department sources said. Sources said the department is determined to protect the forests in and around the city since they are the 'lungs' which can keep the gateway to the North-east green and pollution-free.