WASHINGTON — Insurgents in Iraq have occasionally intercepted video images sent from American military drones to troops in the field, causing the Defense Department to increase its use of encryption over the last year, military and intelligence officials said Thursday.

The military has made extensive use of the remotely piloted drones for surveillance in Iraq and Afghanistan, using live video images to track insurgents, to catch them burying roadside bombs or to identify their houses or weapons caches. Most of that data is highly encrypted, and it has been critical to guiding attacks on the insurgents, often with missiles fired from the drones themselves or from helicopters.

But The Wall Street Journal reported Thursday that Iraqi insurgents had used a $26 commercial software package called Sky Grabber, designed to use a satellite dish to intercept transmissions of movies and music, to capture some of the video feeds from unmanned aerial vehicles.

Military officials confirmed that they had become aware of the intercepts last year. But they said that the vulnerability applied only to a subset of transmissions that had been left unencrypted, and that they had no indication that insurgents had gained much.

They said the vulnerable transmissions occurred when troops with older laptops or handheld controllers sought a direct feed from Predators and smaller surveillance drones, as well as from some conventionally piloted aircraft equipped for surveillance.

Direct video feeds to the troops have proliferated as the military tries to rush the latest intelligence to even the smallest units in the field, and they are expected to play an important role in Afghanistan.

But military officials added that the insurgents would need to be positioned close to the American troops to intercept the feeds.

They said the newest laptops received encrypted signals, just like all the major command centers that receive the main feeds from the largest drones. They said those transmissions had not been compromised.

The officials said they had also been adding encryption, which scrambles the video signal, and taking other steps to reduce the vulnerability of some of the older systems. “But that is a major undertaking, considering that we have hundreds of U.A.V.’s and hundreds more ground stations,” one official said.

Geoff Morrell, the Pentagon press secretary, would not speak specifically about the interception of the video feeds from unmanned aerial vehicles. But he said Pentagon officials “constantly evaluate our weapons platforms, including U.A.V.’s, to make sure that they are performing to maximum effect and that any potential weaknesses and vulnerabilities are mitigated.”

Military officials were always aware of the possibility that unencrypted signals from drones might be intercepted, but initially chose not to encrypt the more localized links to avoid the additional cost, complexity and weight of the necessary equipment, said Dale W. Meyerrose, a retired Air Force officer who worked on the military drone program and later was a senior intelligence official.

“The communications involving high-value intelligence are indeed encrypted,” said Mr. Meyerrose, vice president for cyber initiatives at Harris Corporation.

Officials familiar with the separate Central Intelligence Agency drone program, which has killed militants with Al Qaeda and the Taliban in Pakistan, said it always used encryption.

Officials familiar with the technology said the feeds could show what aspects of insurgent activity were being watched by allied forces, but would not typically reveal the location of Western troops. “I’m really not that concerned,” said one Air Force official of the intercepts.