Russian troops in Ukraine have relied, with surprising frequency, on unsecured communication devices such as smartphones and push-to-talk radios, leaving units vulnerable to targeting, and further underscoring the command-and-control deficiencies that have come to define Moscow’s month-long invasion, observers say.

“We’re seeing them use a lot more unclassified communications because their classified communications capability … for one reason or another, is not as strong as it should be,” a senior U.S. defense official, speaking on the condition of anonymity under terms set by the Pentagon, told reporters in a recent news briefing.

The Russian military possesses modern equipment capable of secure transmission, but troops on the battlefield have reached for simpler-to-use but less-secure lines because of uneven discipline across the ranks, an apparent lack of planning for conducting a sustained fight over long distances, and Russian attacks on Ukraine’s communication infrastructure that it, too, has relied on, experts say.

A European intelligence official, speaking on the condition of anonymity to discuss NATO’s battlefield assessments, said that since the invasion began in late-February, there have been multiple instances of Russian commanders confiscating their subordinates’ personal phones for fear they would unwittingly give away a unit’s location.

Similarly, Ukrainian civilians have reported having their phones stolen by Russian troops who use them to speak with one another and with family back home, this official said. Those calls, the official noted, have revealed troops’ frustrations and declining morale as Ukraine’s military has stymied Russia’s advance around key cities, killing thousands of Russians in the process.

The Pentagon on Friday said that its latest intelligence showed Russian forces had lost full control of Kherson, a port city along the Black Sea, as Ukraine expands its offensive operations in key part of the country and Russia appears to be have shifted its emphasis to the separatist Donbas region in the east. Ukrainian forces also have pushed back Russian advances outside the northern city of Chernihiv with other offensives underway in the western suburbs of Kyiv, the capital, the senior U.S. defense official said.

There is evidence that the United States and other NATO countries have provided Ukrainian forces with electronic warfare equipment capable of interrupting Russian transmissions and allowing them to target Russian command posts, said Kostas Tigkos, a Russian military expert at the defense analysis firm Janes Group. By destroying Russia’s communication nodes, the Ukrainians could pressure their adversaries to use less-secure equipment, he said, increasing the likelihood their conversations will be intercepted or their positions triangulated.

While the Russian military has overhauled its military technology in the last two decades, with some emphasis on modernizing its communication hardware, Tigkos said equipment is only part of the equation. “It’s one thing,” he said, “to develop a good radio that works well. It’s another thing to deploy that radio, build a network, and conduct a complex military operation with thousands of moving parts, and have them work together like a symphony.”

Russian military transmissions over unsecured lines have been so prevalent, analysts say, that amateur radio enthusiasts have tuned into them online using sites such as Web SDR. Some conversations have revealed troops’ frustrations. In one transmission on March 5, a Russian service member identifies himself as “Blacksmith,” rather than a call sign. “Don’t say the last names on air!” another responds. The transmission was provided to The Washington Post by Shadow Break International, an open-source intelligence consultancy based in Britain.

Audio shared with The Washington Post by Shadow Break International showed Russian soldiers speaking in unsecure radio transmissions in Ukraine on March 5. (Video: The Washington Post, Photo: The Washington Post)

In another discussion, Russian soldiers appear to confuse one another by mistaking their callsigns. One identifies himself as “Exchange.” Another then says that, in fact, that’s his call sign. “You got it all mixed up!” one of them explains.

Russian commanders also have exhibited difficulty orchestrating communications over such a vast, dynamic battlefield, analysts say. Their forces are stretched across Ukraine, the largest country in Europe outside Russia, posing challenges for military planners who must coordinate mobile transmission sites and ensure radios are operating on frequencies that must be consistently changed.

At the same time, military analysts have cautioned against making sweeping generalizations of the Russians’ communication performance. Some units, they say, may be better equipped and disciplined than others.

Photos of Russian equipment captured by Ukrainian forces show sophisticated and secure radios, said Sam Bendett, a Russian military technology expert at the Center for Naval Analyses in Arlington, Va. Other imagery show off-the-shelf equipment. Some Russian personnel may use such radios as a means to blend into the wide spectrum of civilian frequencies — like a needle in a haystack, Bendett said — rather than military frequencies that are more limited and detectable with the right equipment.

There is anecdotal evidence that Russia’s unsecured communications have led to battlefield losses. One Russian general was purportedly killed in an airstrike after his cellphone was detected by the Ukrainians, the New York Times reported earlier this month.

In another instance, shared by the Ukrainian Defense Ministry, two Russian intelligence officials were heard discussing over an open frequency the death of a senior officer. When one asked to speak on an encrypted line, the other said it wasn’t working.

“We can’t get in touch with anyone at all,” the official said, lamenting his inoperable phone, called the Era. The Russian-made device relies on a cellular network to function, but heavy bombardment has destroyed cell towers in many parts of the country, in turn constraining the Russians’ ability to use secure phones, said Tigkos, the analyst with Janes Group.

It’s also likely that senior Russian officers with experience battling less capable forces in other theaters had become somewhat complacent and were caught off guard by how determined Ukrainian forces have proven to be. Russian commanders have rotated through Syria for years, where radios and cellphones could be used without worry of interference or tracking, noted Bendett, of the Center for Naval Analyses.

“It appears likely,” he said, “some officers picked up bad habits that they thought would work in Ukraine.”

Joyce Sohyun Lee and Dan Lamothe contributed to this report.