Ukrainian officials claimed on Friday they used a barrage of drones to destroy at least six military aircraft and badly damage eight others at an airfield in Russia’s Rostov region. Russian defense officials, however, claimed they intercepted 44 Ukrainian drones and that only a power substation was damaged in the attack.

The Associated Press could not independently verify either side’s claims.

The assault appeared to be one of Kyiv’s biggest air attacks in the war, coming as its forces stepped up their assaults on Russian soil. Russia has also escalated attacks on civilian infrastructure, including Ukraine’s power plants, in recent weeks, signalling a new and potentially dangerous phase in the conflict as both sides struggle to achieve significant advances on the ground.

The overnight attack targeted a military airfield near Morozovsk in Russia and was conducted by Ukraine’s Security Service in cooperation with the army, Ukrainian intelligence officials told the AP.

They said around 20 members of the airfield’s personnel were killed or injured.

Morozovsk airfield was used by Russian bombers that have been launching guided aerial bombs at Ukraine’s cities and frontline positions, the officials said. They spoke on condition of anonymity because they were not authorized to publicly discuss the operation.

If true, the attack would be among Ukraine’s most successful cross-border strikes. Last October, Ukraine claimed it destroyed nine Russian helicopters at two airfields in Russian-occupied regions using longe-range ballistic missiles donated by the US.

Last August, Ukrainian media, citing unidentified intelligence sources, claimed that drone attacks hit parked Russian bomber aircraft at air bases deep inside Russia.

In a conflicting version of events, Russia’s defence ministry said a total of 44 drones were “intercepted and destroyed” in the Morozovsky district, more than 100 km from the border. The attack damaged a power substation, Rostov governor Vasily Golubev said, adding that eight people near the airfield were injured.

Usually well-informed Russian military bloggers confirmed an attempted attack by Ukrainian drones on a military air base in Morozovsk but claimed there were no casualties at the base and no damage to warplanes. The Russian defence ministry said nine more drones were intercepted over the border regions of Kursk, Belgorod, Krasnodar and the nearby Saratov region, bringing the total of attack drones deployed by Ukraine to 53.

Drone warfare is a key feature of the war, which has extended into a third year since Russia’s full-scale invasion of its neighbour.