ISLAMABAD – The Taliban's top military commander has been arrested in a joint CIA-Pakistani operation in Pakistan in a major victory against the insurgents as US troops push into their heartland in southern Afghanistan, officials said on Tuesday. Mullah Abdul Ghani Baradar, the group's No. 2 leader behind Afghan Taliban founder Mullah Mohamed Omar and a close associate of Osama bin Laden, was captured in the southern Pakistani port city of Karachi, two Pakistani intelligence officers and a senior US official said. They spoke on condition of anonymity because they were not authorised to release such sensitive information. One Pakistani officer said Baradar was arrested 10 days ago with the assistance of the United States and "was talking" to his interrogators. Baradar is the most senior Afghan Taliban leader arrested since the beginning of the Afghan war in 2001 following the September 11 terror attacks in the United States. Meanwhile, a NATO airstrike against suspected insurgents in Kandahar has instead killed five civilians, officials said. The group was seen digging on a roadside and was thought to be planting bombs, NATO said. A senior official said he regretted the loss of life, adding that an investigation was underway. The incident was unrelated to the ongoing operation against the Taliban in Helmand. A NATO rocket strike there killed 12 civilians on Sunday. Civilian casualties are particularly sensitive during the joint NATO and Afghan Operation Moshtarak to force the Taliban out of their strongholds in Helmand. Maj Gen Michael Regner said of the Kandahar deaths: "We regret this tragic accident and offer our sympathies to the families of those killed and injured.” "Our combined forces take every precaution to minimise civilian casualties and we will investigate this incident to determine how this happened." Operation Moshtarak, meaning "together" in the Dari language, is the biggest coalition attack since the Taliban fell in 2001. The operation is also considered the first big test of US President Barack Obama's new "surge" strategy for Afghanistan.