US Secretary of State John Kerry has said, terror groups and extremism cannot be fought by one nation and all countries have to confront the menace together. Interacting with the students of IIT Delhi this morning, Mr Kerry said, both India and US know the pain of terrorism and it is important to strike its root cause. He said, security threat is the biggest challenge.<br/><br/>Speaking on the Indo-US relationship, the Secretary of State said, in recent years, trade between the two nations has doubled and Washington is now New Delhi's top export market. He said, both countries worked together in Nepal during the devastating earthquake, evacuated the people trapped in war-torn Yemen and trained peace-keepers in Africa.<br/><br/>Talking on India, John Kerry said, India is an established power and is playing a crucial role in the world today. He said, India is an essential partner and the US has welcomed its rise. Both countries are the two largest democracies in the world bound with common values for tolerance, equality, respect of religious pluralism and peace.