WASHINGTON » A former legal adviser to the State Department has sharply criticized the secrecy surrounding the Obama administration’s use of drones for the targeted killing of terrorism suspects, saying it is unnecessary and has backfired.

In a speech Tuesday at Oxford University, Harold H. Koh, a Yale law professor who as the State Department’s top lawyer in 2010 gave the first public speech explaining the targeted killing program, said the administration "has not been sufficiently transparent to the media, to the Congress and to our allies."

The result is "a growing perception that the program is not lawful and necessary, but illegal, unnecessary and out of control," said Koh, who served during President Barack Obama’s entire first term. "The administration must take responsibility for this failure, because its persistent and counterproductive lack of transparency has led to the release of necessary pieces of its public legal defense too little and too late."

Koh defended the drone program but said it should be "disciplined," and his speech came as the White House works on changes that Obama has suggested will bring both greater transparency and a clearer legal framework to the program. The details are still unknown, though some officials have said that the Defense Department may take over the leading role on drones from the CIA, and that so-called signature strikes, in which the identity of the people targeted is unknown, will be eliminated.

In March, a former top Pentagon lawyer, Jeh C. Johnson, similarly appealed for greater openness about the drone program in a public speech, though he also cautioned that "the reality is that it is much easier to classify something than to declassify it."

Despite the promises of openness that Obama has made in interviews and in his State of the Union address in February, the administration has continued to enforce strict secrecy on the drone program. The White House shared classified legal opinions on the targeted killing of Americans with the Senate and House Intelligence Committees only when it seemed necessary to win confirmation for John O. Brennan, Obama’s counterterrorism adviser, as CIA director. Last month, Obama rebuffed repeated requests from a Senate committee to allow an administration official to testify at a hearing on the legal and policy issues raised by drone strikes.

Koh said the administration should make public the legal standards and procedures for choosing targets and carrying out strikes, against both foreigners and Americans overseas. It should clarify its method for counting civilian casualties in the strikes and release "the factual record" of disputed past strikes to explain the rationale for them, he said.

The administration should consult with allies about the strikes "to reassure them that we are not applying a standard that we would consider unlawful if espoused to justify the use of drones by, say, China, North Korea or Iran," he said.

In a clear reference to the snub of the drone hearing, Koh said that "the administration should send witnesses to explain its legal standards to Congress" and "patiently explain why the use of force was warranted in particular, well-publicized cases."

Koh praised the president’s recent repeating of his longstanding call for the closing of the prison at Guantanamo Bay, Cuba, and he set out several steps Obama could take to start the process, beginning with the appointment of a top White House official to work exclusively on that goal. Closing Guantanamo is one of several major actions the U.S. could take, Koh said, to convince Americans and the world that what he called "the forever war" following the 2001 terrorist attacks will in fact be brought to a close.

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