WASHINGTON -- The United Nations opened a major new investigation on Thursday into the United States' use of drones and targeted assassinations.

The U.N. investigation, led by special rapporteur on counterterrorism and human rights Ben Emmerson, is expected to focus on the legal justification for America's expansive drone program, which has largely remained secretive and unexamined.

"The exponential rise in the use of drone technology in a variety of military and non-military contexts represents a real challenge to the framework of established international law," Emmerson said in a statement released by his office.

"It is therefore imperative that appropriate legal and operational structures are urgently put in place to regulate its use in a manner that complies with the requirements of international law, including international human rights law, international humanitarian law (or the law of war as it used to be called), and international refugee law."

Human rights observers have long objected to the use of drones to target suspected terrorists because they often result in wider civilian deaths than administration officials have acknowledged.

But more practical concerns -- about the legality and efficacy of the program, as well as the White House's lack of transparency -- have also been growing.

The United States is by far the leading user of drones and unmanned vehicles for targeted assassinations, but it is not the only one. Drone use is expected to expand widely around the world -- China and Iran already are known to have the capability -- and there have been growing calls for the U.S. to clarify its own internal rules for the appropriate use of the technology.

In a recent conference call organized by the Council on Foreign Relations this week, Dennis Blair, the former director of national intelligence under President Barack Obama, urged the administration to make more of its drone policies public.

"There's been far too little debate" about the tactic, Blair said in the call. "The United States is a democracy, we want our people to know how we use military force and that we use it in ways the United States is proud of."

The American Civil Liberties Union, which has been waging a years-long effort to compel the Obama administration to release its internal legal considerations, welcomed the U.N. investigation, and urged the U.S. to participate in it.

“Virtually no other country agrees with the U.S.’s claimed authority to secretly declare people enemies of the state and kill them and civilian bystanders far from any recognized battlefield," said Hina Shamsi, the director of the ACLU’s National Security Project, in a statement. "To date, there has been an abysmal lack of transparency and no accountability for the U.S. government’s ever-expanding targeted killing program.”

Also on Thursday, the American Security Project released a new report examining the efficacy of drone strikes and raising questions about the strategy of using unmanned aerial vehicles for counterterrorism.

The report argues that the apparent tactical success of drones -- their ability to kill suspected terrorists without significant risk to American forces -- does not answer important questions about the strategic benefits of the program, or the broader strategic thinking behind it.

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  • Lahore

    Pakistani NGOs workers shout slogans against US drone attacks and religious fundamentalism during a protest in Lahore on October 21, 2010. (Arif Ali/AFP/Getty Images)

  • Manila

    Protesters set on fire to a mock model of a U.S. drone during a rally near the U.S. Embassy in Manila, Philippines, on Friday Jan. 11, 2013. They protested an unarmed target drone found in central Philippine waters over the weekend which U.S. officials claimed was launched from a U.S. Navy ship during a combat exercise off Guam last year and may have been washed by ocean currents to the country. The embassy spokeswoman Bettina Malone said the BQM-74E drone was launched from the USS Chafee, a guided-missile destroyer, as a mock missile target during naval combat exercises off Guam. (AP Photo/Bullit Marquez)

  • Islamabad

    American anti-war coalition CodePink activists, protest while fasting to condemn U.S. drone attacks in Pakistani tribal region, Tuesday, Oct. 9, 2012 in Islamabad, Pakistan. The placard, center, reads, "fasting for peace." (AP Photo/B.K. Bangash)

  • New York

    Nick Mottern of Hastings-on-Hudson, N.Y., hands out information to a passerby as he stands beneath a model of an unmanned drone, which he labeled an "unmanned assasination vehicle" during an anti-war teach-in as part of the Occupy Wall Street protest now in its fourth week at Zuccotti Park in New York, Tuesday, Oct. 11, 2011. (AP Photo/Kathy Willens)

  • Washington DC

    A cardboard 'drone' is seen at an occupy DC camp in Freedom Plaza in Washington, DC. Occupy DC's website states the movement is built on the example of Occupy Wall Street, whose activists have continuously camped out in a New York park since September 17. (KAREN BLEIER/AFP/Getty Images)

  • Lahore

    Activists of the Pakistani fundamentalist Islamic party Jamaat-i-Islami (JI) shout slogans against the release of CIA contractor Raymond Davis and an US drone strike in the Pakistani tribal area, during a protest rally in Lahore on March 18, 2011. (Arif Ali/AFP/Getty Images)

  • Islamabad

    A Pakistani boy holds a placard during the second day of protests against the US drone attacks along with tribesmen of north Waziristan in Islamabad on December 10, 2010. (FAROOQ NAEEM/AFP/Getty Images)

  • Washington DC

    Members of Pax Christi USA, Foreign Policy in Focus, CODEPINK and other organizations, mock and protest the unmanned US drone attacks in Afghanistan and Pakistan on October 7, 2010 at Union Station in Washington DC. (KIMIHIRO HOSHINO/AFP/Getty Images)

  • Multan

    Pakistani demonstrators shout anti-US slogans during a protest in Multan on January 8, 2013, against the drone attacks in Pakistan's tribal areas. (S.S MIRZA/AFP/Getty Images)

  • Islamabad

    American citizens rally in Islamabad, Pakistan against drone attacks in Pakistani tribal belt, Friday, Oct. 5, 2012. (AP Photo/B.K. Bangash)

  • North Carolina

    Protesters march with a drone effigy outside Duke Energy headquarters in Uptown, the Charlotte the business district, before the start of the Democratic National Convention (DNC) September 2, 2012 in Charlotte, North Carolina. (Photo by Tom Pennington/Getty Images)

  • Florida

    Thomas Bolanos holds an American flag as he joins others in a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Largo, Florida. (Photo by Joe Raedle/Getty Images)

  • Florida

    Dina Formentini (C) and Code Pink leader Medea Benjamin (R) join others in a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Largo, Florida. (Photo by Joe Raedle/Getty Images)

  • Florida

    Code Pink leader Medea Benjamin holds a sign reading, 'Raytheon's Drones Create Enemies,' as she joins others in a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Largo, Florida. (Photo by Joe Raedle/Getty Images)

  • Florida

    Clay Colson holds a sign reading, ' Healthcare not Warfare!', as he joins others in a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Largo, Florida. (Photo by Joe Raedle/Getty Images)

  • Florida

    Code Pink leader Medea Benjamin holds a sign reading, 'Raytheon's Drones Create Enemies,' as she listens to a police officer ask the group to move to a public sidewalk durin a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Largo, Florida. (Photo by Joe Raedle/Getty Images)

  • Florida

    Liz, who only wanted to be identified by her first name, holds a sign reading, 'Obama's Drones Kill Americans, Too,' as she joins others in a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Largo, Florida. (Photo by Joe Raedle/Getty Images)

  • Florida

    James L. holds a sign reading, ' Obama is just another war prez ', as he joins others in a protest in front of a Raytheon company building which they say is building military drones on August 23, 2012 in Tampa, Florida. (Photo by Joe Raedle/Getty Images)

  • Philippines

    Protesters display placards during a rally near the U.S. Embassy in Manila, Philippines, on Friday Jan. 11, 2013. (AP Photo/Bullit Marquez)

  • Philippines

    Protesters march towards the U.S. Embassy in Manila with a mock model of a U.S. drone Friday Jan. 11, 2013. (AP Photo/Bullit Marquez)

  • YEMEN-UNREST-DEMO-DRONE

    Yemenis hold up a sign in Arabic that reads, 'No to Foreign Intervention...No to American Terrorism' during a protest against US drone attacks on Yemen close to the home of Yemeni President Abdrabuh Mansur Hadi, in the capital Sanaa, on January 28, 2013. Strikes by US drones in Yemen nearly tripled in 2012 compared to 2011, with 53 recorded against 18, according to the Washington-based think-tank New America Foundation. AFP PHOTO/STR (Photo credit should read -/AFP/Getty Images)

  • YEMEN-UNREST-DEMO-DRONE

    A Yemeni hold up a banner during a protest against US drone attacks on Yemen close to the home of Yemeni President Abdrabuh Mansur Hadi, in the capital Sanaa, on January 28, 2013. Strikes by US drones in Yemen nearly tripled in 2012 compared to 2011, with 53 recorded against 18, according to the Washington-based think-tank New America Foundation. AFP PHOTO/STR (Photo credit should read -/AFP/Getty Images)