Ukraine’s most sophisticated attack drone is about as stealthy as a crop duster: slow, low-flying and completely defenseless. So when the Russian invasion began, many experts expected the few drones that the Ukrainian forces managed to get off the ground would be shot down in hours.

But more than two weeks into the conflict, Ukraine’s drones — Turkish-made Bayraktar TB2 models that buzz along at about half the speed of a Cessna — are not only still flying, they also shoot guided missiles at Russian missile launchers, tanks and supply trains, according to Pentagon officials.

The drones have become a sort of lumbering canary in the war’s coal mine, a sign of the astonishing resiliency of the Ukrainian defense forces and the larger problems that the Russians have encountered.

“The performance of the Russian military has been shocking,” said David A. Deptula, a retired three-star Air Force general who planned the U.S. air campaigns in Afghanistan in 2001 and the Persian Gulf in 1991. “Their failure to secure air superiority has been reflected by their slow and ponderous actions on the ground. Conversely, the Ukrainian air force performing better than expected has been a big boost to the morale of the entire country.”

The people of Ukraine are singing songs about the Bayraktar drone and repeatedly posting online footage of destroyed Russian armor. They have given the name Bayraktar to a lemur born last week at the zoo in Kyiv, the capital.

A senior Pentagon official confirmed that Ukrainian forces had successfully used armed Bayraktars to carry out several attacks on the huge Russian military convoy that has been making its way toward Kyiv. The drones have also been used for reconnaissance, hunting for targets for Ukrainian ground troops. The Pentagon official said he could not confirm the authenticity of videos posted online that purported to show Bayraktar airstrikes.

Before Russia invaded Ukraine, Bayraktar TB2s were already punching above their weight. The drones, with a 39-foot wingspan, are assembled in Turkey but rely extensively on electronics made in the United States and Canada. A growing number of countries in Africa, the Middle East and Europe have bought them because, at about $2 million apiece, they are much cheaper than manned combat aircraft.

In recent years, TB2s have been used to attack targets in Syria, Libya and Nagorno-Karabakh — each time against opponents armed with Russian-made tanks and antiaircraft systems, and each time landing devastating blows on enemy ground forces.

But military planners and civilian experts cautioned that the drones — which have no self-defense systems, are easily spotted by radar and cruise at only about 80 miles an hour — would be sitting ducks for Russia’s many-layered air defense system. Russian forces have long-range cruise missiles that can destroy the drones on the ground, short-range missile systems that can easily knock them out of the air, and electronic jammers that can block the drones’ communications, leaving them to drop lifeless from the sky.

“Even with the drones’ record of success, everyone expected that, once they really faced the full gamut of Russian defenses, they would stand no chance,” said Lauren Kahn, who studies drone warfare at the New York-based Council on Foreign Relations. Their survival and continued use “is really raising questions about the Russians’ capabilities,” she said.

Pentagon officials remain puzzled by the Russians’ failure to dominate the skies over Ukraine, at least so far. Moscow built up sophisticated missile defenses and air power on Ukraine’s borders, but it has not been using them effectively to complement its ground forces, U.S. officials and analysts said. And Ukrainian air defenses have been surprisingly effective at downing Russian aircraft.

“We aren’t seeing the level of integration between air and ground operations that you would expect to see,” John F. Kirby, the chief Pentagon spokesman, said on Monday. “Not everything they’re doing on the ground is fully being supported by what they’re doing in the air. There does seem to be some disconnect there.”

Ukraine’s success at keeping Russia from dominating its airspace not only allows the country to fly its drones, it also limits Russia’s ability to send drones to hunt for the small teams of Ukrainian ground troops who have used shoulder-fired missiles and other weapons to knock out hundreds of Russian vehicles.

“It is so perplexing, and no one is quite sure what went wrong,” said Samuel Bendett, an expert on the Russian military at the Center for a New American Security, a Washington-based research group. “Russia has a large number of drones, and the assumption was they would be using them for strikes,” he said. “That assumption has been completely undone.”

The Russian forces seemed to be using drones very little so far, Mr. Bendett said, perhaps because they are afraid the drones will be shot down with Ukraine’s air space still contested.

Without air superiority, the Russian offensive has been bogged down, claiming little new territory in recent days while losses mount. The Pentagon estimated on Wednesday that 5,000 to 6,000 Russian troops had been killed, and observers said the number of tanks, missile launchers and trucks that Russia had lost ran into the hundreds.

At the start of the war, Ukraine had five to 20 Bayraktar TB2s in service. Russia claims to have shot down several of them, and it is unclear how many remain. Still, Ukraine continues to release video images that appear to show the drones destroying Russian vehicles.

Air superiority is seen as a critical first step in modern warfare, and armed forces spend a great deal of time and money trying to ensure that they can quickly dominate the skies when fighting starts. Strategists studying Russia assumed that it would immediately use missile strikes to destroy Ukraine’s air force and surface-to-air missile batteries before they could be used, and then move in scores of fighter jets, radar jammers and missile trucks to take control of Ukraine’s air space. With air superiority established, Russia could freely use its fighters, bombers and drones to annihilate the Ukrainian military.

That has not happened.

In the first days of the invasion, the Russian military appeared to hold back much of its air power, perhaps assuming that the Ukrainian military would not put up much of a fight. Instead, Russian forces met stiff resistance; when they tried to move in mobile missile launchers and electronic warfare vehicles to control the airspace, the convoys were ambushed by Ukrainians before they could reach the fight.

“It’s certainly not the way we would prosecute an air campaign,” said Michael Kofman, the director of Russia studies at C.N.A., a defense research institute in Arlington, Va.

“But then again, this war didn’t start the way the Russian military organizes and trains to fight, either,” he said. “It was a bungled regime-change operation that became a war they didn’t really plan for.”

But lack of a quick victory for Russia did not mean victory for Ukraine, Mr. Kofman added, noting that Ukraine continues to lose aircraft to Russian missiles, and that it was not possible to glean the true state of the air war from official statements and news reports alone.

Paradoxically, experts say, Ukraine’s early success in the skies may only prolong the war and increase the destruction, as the much larger Russian military appears to be shifting from precision strikes to widespread shelling and bombing of civilian neighborhoods.

Russia is believed to still have forces in reserve that it could use to try to establish the air superiority that it fumbled at the start. Defense officials say few Ukrainian aircraft are now flying, and they must pick their targets carefully to avoid areas where strong Russian defenses might shoot them down.

For the time being, Ukraine’s drones are still flying, U.S. defense officials say. And on Thursday, video footage appeared on social media claiming that one of the Bayraktar drones had destroyed a Russian mobile missile launcher — exactly the kind of expensive, sophisticated weapon system that Russia fielded to wipe out the inexpensive drones that had destroyed it instead.