A major Pakistani military offensive in the northwest has killed up to 700 militants in the past four days, and the operation will proceed until the last Taliban fighter in the area is ousted, the country's top civilian security official said Monday. The offensive in Swat and surrounding districts has earned praise from the US, which wants al-Qaida and Taliban militants rooted out from Pakistani havens where they can plan attacks on American and NATO forces in Afghanistan. But the fighting has unleashed an exodus of refugees, and raised concern over the future of nuclear-armed Pakistan. Interior Minister Rehman Malik's announcement of 700 militants killed came as a witness and a police official reported new airstrikes in parts of the Swat Valley, a one-time tourist haven that fell prey to Taliban advances two years ago. Malik's toll - which exceeds that given by the military on Sunday by at least 200 - could not be independently confirmed.