One of the Taliban's top leaders has been captured in Pakistan in a joint operation by Pakistani and American intelligence operatives, U.S. officials said Monday.

The man, Mullah Abdul Ghani Baradar, the group's military chief, is the highest-ranking Taliban figure to be caught since the 2001 invasion of Afghanistan. He effectively functions as the Afghan Taliban's second in command, and his capture represents a major victory in the battle against the militants—and the effort to secure greater Pakistani cooperation in the fight.

Pakistan's Inter-Services Intelligence spy agency, which U.S. officials have long complained aids some elements of the Taliban, led the operation to capture Mr. Baradar, who was detained in the Arabian Sea port city of Karachi.

Mr. Baradar is being questioned by the ISI with the CIA's involvement, and a U.S. official in the region said his capture had proved "really valuable." The official wouldn't elaborate. He has been in custody for about two weeks and "is providing intelligence information," another American official said. "It's a very significant takedown."

Taking Mr. Baradar down "would deal a major setback to the Afghan Taliban and be a personal blow to Mullah Omar, who has relied heavily on him for years," a U.S. counterterrorism official said.