Al Qaeda's operations chief in Pakistan and a top aide are believed to be dead, a U.S. counterterrorism official said on Thursday, in what appeared to be the latest results of a campaign targeting the militant group's leadership.

Operations chief Usama al-Kini was thought responsible for attacks, including the bombing of a Marriott hotel in Islamabad that killed 55 people in September, and an unsuccessful attempt to kill former Pakistani Prime Minister Benazir Bhutto, who was later assassinated in a separate attack, the official said.