GENEVA, Switzerland - The escalating campaign of CIA drone strikes against suspected terrorists in Pakistan has made the United States "the most prolific user of targeted killings" in the world today, according to a U.N. official who said the spy agency should not be in charge of the program.

Philip Alston, a New York University law professor and the United Nations' special rapporteur on extrajudicial killings, made the comments Wednesday as he released a report on targeted killings that criticized the United States for asserting "an ever-expanding entitlement for itself to target individuals across the globe" as part of its antiterror fight.

The findings, which Alston is due to present Thursday to the U.N. Human Rights Commission in Geneva, form one of the most critical assessments to date of U.S. drone strikes, a tactic that has been stepped up significantly under the Obama administration and that U.S. officials have credited with inflicting severe blows against al-Qaeda and other terrorist groups.

As the drone attacks have expanded, they have drawn increasing criticism from human-rights groups and international legal scholars. Some say aspects of the program violate international law and risk generating a backlash in Pakistan and other countries where the strikes are carried out.

Although the United States does not officially acknowledge the CIA drone strikes, much of the report was dismissed by Obama administration officials.

One U.S. counterterrorism official who was not authorized to comment publicly, and spoke on condition of anonymity, said: "We have a way to get at dangerous terrorists operating in areas otherwise inaccessible to the central government or to conventional military units. It's effective, exact, and essential."

CIA spokesman Paul Gimigliano said in a statement that the agency's operations "are, of course, designed to be lawful and are subject to close oversight within our government. The accountability is real, and so is the fidelity to American policy."

White House deputy press secretary Bill Burton would not comment on the U.N. report's findings, but he said Obama "is focused on making sure that he's doing everything in his power to protect the security of our country."

Alston's report discusses targeted killings by several countries, including Russia, Israel, and Sri Lanka. But he focused on the U.S. use of drones, arguing that other governments are likely to copy the tactic in coming years.

Many countries seek to acquire unmanned aircraft, the report said, because the drones allow "targeted killing at little or no risk" and can be operated by pilots thousands of miles away.

"This strongly asserted but ill-defined license to kill without accountability is not an entitlement which the United States or other states can have without doing grave damage to the rules designed to protect the right to life and prevent extrajudicial executions," Alston said.

He said the more widespread use of drone attacks could undermine international human-rights rules that have been interpreted to govern the use of lethal force.

Current and former U.S. officials familiar with the program took issue with Alston's findings, including the assertion that the White House is claiming "an ever-expanding" right to conduct drone strikes anywhere in the world.

In fact, two officials said, the Obama administration has limited who may be targeted in drone strikes outside of Afghanistan and Pakistan to members of al-Qaeda and allied terror groups - a tighter standard than existed during the Bush administration.

U.S. officials say that careful precautions are taken to avoid civilian casualties and that each target is carefully vetted, sometimes by hours of aerial surveillance that is matched against other intelligence to create a portrait of a potential target, known as a "pattern of life" analysis.

Such air strikes, officials say, are one of the few viable options for going after extremists who have taken refuge in the lawless border region.

Alston called for greater openness by the United States about the program, including disclosure of the identities of those killed. He also called on the Obama administration to turn over the mission of conducting air strikes against suspected terrorists to the U.S. military, which he said was more accountable than the CIA and likely to be better trained in the law of warfare.