The Centre has issued an alert to heighten security at major airports and metro networks across the country in the wake of Brussels terror attacks.<br/><br/>In a communication, the central government today asked the state governments and security agencies, to beef up security at airports in Delhi, Mumbai, Chennai, Bengaluru, Hyderabad and Kolkata.<br/><br/>According to PTI sources, Instructions have also been given to tighten security in metro networks in Delhi, Kolkata, Mumbai and Bengaluru.