The White House is preparing a directive that would require federal agencies to publicly disclose for the first time where they fly drones in the United States and what they do with the torrents of data collected from aerial surveillance.

The presidential executive order would force the Pentagon, the Justice Department, the Department of Homeland Security and other agencies to reveal more details about the size and surveillance capabilities of their growing drone fleets — information that until now has been largely kept under wraps.

The mandate would apply only to federal drone flights in U.S. airspace. Overseas military and intelligence operations would not be covered.

President Obama has yet to sign the executive order, but officials said that drafts have been distributed to federal agencies and that the process is in its final stages. “An interagency review of the issue is underway,” said Ned Price, a White House spokesman. He declined to comment further.

Privacy advocates said the measure was long overdue. Little is known about the scope of the federal government’s domestic drone operations and surveillance policies. Much of what

has emerged was obtained

under court order as a result of public-records lawsuits.

“We’re undergoing a quiet revolution in aerial surveillance,” said Chris Calabrese, legislative counsel for the American Civil Liberties Union. “But we haven’t had all in one place a clear picture of how this technology is being used. Nor is it clear that the agencies themselves know how it is being used.”

Most affected by the executive order would be the Pentagon, which conducts drone training missions in most states, and Homeland Security, which flies surveillance drones along the nation’s borders round-the-clock. It would also cover other agencies with little-known drone programs, including NASA, the Interior Department and the Commerce Department.

Military and law enforcement agencies would not have to reveal sensitive operations. But they would have to post basic information about their privacy safeguards for the vast amount of full-motion video and other imagery collected by drones.

Until now, the armed forces and federal law enforcement agencies have been reflexively secretive about drone flights and even less forthcoming about how often they use the aircraft to conduct domestic surveillance.

Security officials are generally reluctant to disclose operational methods and techniques. But drones are in a special category of sensitivity, given the top-secret role they’ve long played in CIA and military counterterrorism missions. There’s also evidence that federal agencies simply have been unable to develop internal guidelines and policies quickly enough to keep up with rapid advances in drone technology.

“Federal use of drones has gone way up, but it’s hard to document how much,” said Jennifer Lynch, a lawyer with the Electronic Frontier Foundation, a San Francisco-based group that has sued the Federal Aviation Administration for records on government drone operations. “It’s been in­cred­ibly difficult.”

Even Congress has struggled to uncover the extent to which the federal government uses drones as a surveillance tool in U.S. airspace.

In March 2013, lawmakers directed the Defense Department to produce a report, within 90 days, describing its policies for sharing drone surveillance imagery with law enforcement agencies.

Eighteen months later, the Pentagon still has not completed the report. Air Force Lt. Col. Thomas Crosson, a Defense Department spokesman, said officials hoped to provide an interim response next week and a full version “in the coming months.”

Department of Justice officials have also been reluctant to answer queries from lawmakers about their drone operations. The FBI first disclosed its use of small, unarmed surveillance drones to Congress in June 2013 and subsequently revealed that it had been flying them since 2006.

The Justice Department inspector general reported last fall that the FBI had not developed new privacy guidelines for its drone surveillance and was relying instead on old rules for collecting imagery from regular aircraft.

Since then, Justice officials have said they are reviewing their drone surveillance policies but have not disclosed any results. An FBI spokesman did not respond to a request for comment.

The FBI has resisted other attempts to divulge details about the size of its drone fleet and its surveillance practices.

Citizens for Responsibility and Ethics in Washington (CREW), a nonprofit group that pushes for transparency in government, sued the FBI last year under the Freedom of Information Act for records on its drone program. Although the FBI has turned over thousands of pages of documents, many have been redacted or provide only limited insights.

“They’ve been dragging their feet from the outset, and it’s been enormously frustrating,” said Anne Weismann, CREW’s chief counsel. “I don’t know if it’s because they don’t want to expose the fact that they’ve been operating without any clear guidance or if they just don’t like to talk about it.”

Another section of Obama’s draft executive order would instruct the Commerce Department to help develop voluntary privacy guidelines for private-sector drone flights. The intent is to shape nonbinding industry standards for commercial surveillance instead of imposing new regulations by law.

The executive order is an attempt to cope with a projected surge in drone flights in the United States.

For years, the FAA has enforced a de facto ban on commercial drone flights. The FAA permits government agencies to fly drones only under tightly controlled circumstances.

Under a 2012 law passed by Congress, however, the FAA is developing rules that will gradually open the skies to drones of all kinds. The drone industry, which lobbied Congress to pass the law, predicts $82 billion in economic benefits and 100,000 new jobs by 2025.

On Thursday, the FAA approved requests from six Hollywood filmmakers to fly small camera-equipped drones on movie sets, the first time businesses will be allowed to operate such aircraft in populated areas. About 40 companies, including Amazon.com, have filed similar requests with the FAA. Amazon’s chief executive, Jeffrey P. Bezos, owns The Washington Post.

Federal lawmakers have introduced several bills in recent years to regulate the use of drones by law enforcement agencies and strengthen privacy protections, but none has passed.

No department flies more drones than the Pentagon, which has about 10,000 of the aircraft in its inventory, from four-pound Wasps to the 15-ton Global Hawk.

While many are deployed overseas, Defense Department documents show that the military is making plans to base drones at 144 sites in the United States. Pentagon officials have said they soon expect to fly more drones in civilian airspace in the United States than in military-only zones.

The Department of Homeland Security also conducts extensive surveillance with unarmed drones. Its Customs and Border Protection service has nine large Predator B models, which account for about three-quarters of all drone flight hours reported by federal civilian agencies.

Customs and Border Protection drones patrol a 25-mile-wide corridor along the nation’s northern and southern borders, as well as over the Caribbean Sea and Gulf of Mexico.

Records obtained by the Electronic Frontier Foundation show that the Border Patrol has also outsourced its drones on hundreds of occasions to other law enforcement agencies throughout the United States. Details of most of those operations remain secret.