Indian Prime Minister Manmohan Singh on Saturday said that terror is the biggest threat in South Asia as the 15th SAARC summit began in Colombo. Terming the twin blasts in Bangalore and Ahmedabad and the Indian Embassy attack in Kabul a gruesome reminders of terrorism, Dr Singh stressed on the need to fight the menace together. He further called on the eight-nation regional grouping to identify and address new challenges if it wants to be a dynamic and responsive body.