John Brennan's speech, on the eve of the one-year anniversary of the U.S. killing of al Qaeda leader Osama bin Laden in a raid in Pakistan, was remarkable in that U.S. officials in public rarely discuss the drone program, which has for years been considered a covert CIA operation.

  The bin Laden commando raid and drone strikes have severely strained U.S. relations with Pakistan. But the frayed ties have not stopped the United States from pursuing militants with the unmanned aircraft that can be remotely piloted from hundreds or thousands of miles away. 

Last weekend, a U.S. drone strike killed four suspected militants in Pakistan's North Waziristan tribal region, despite the recent demand by a Pakistani parliamentary committee that such operations end.

  "I'm here today because President Obama has instructed us to be more open with the American people about these efforts," Brennan said in a speech at the Woodrow Wilson International Center for Scholars, a Washington think tank.