WASHINGTON -- The U.S. believes Central Intelligence Agency drone attacks have killed two prominent Islamic militant figures in Pakistan affiliated with al Qaeda, one of whom was on the U.S.'s list of top 20 targets, according to officials briefed on the matter.

One drone attack Monday is believed to have killed the leader of the Islamic Jihad Union, Najmiddin Kamolitdinovich Jalolov, an Uzbek native implicated in terrorist plots and attacks in Germany and Uzbekistan. Officials said they are almost certain he was killed, though...