Implementation of Security in Agile model for web developement

Hi, I am working in a big team of web application development, and we are facing big challenges in terms of implementation of security in our project. We are using AGILE model for product development and usually we don’t get time in our mid sprints to spend on such issues. It is now becoming very painful for me as a lead to solve the bug related to security in estimated time. Please suggest if any one has implemented basic security steps in parallel to development.