KYIV (Reuters) -Around 10,000 Russian soldiers are fighting in Russia's Kursk region, about 90 square kilometers (35 square miles) of which is controlled by Ukraine, Ukraine's top military commander said.

"We control about 90 square kilometers of territory in the Hlushkov district of the Kursk region of the Russian Federation, and these are our preemptive actions in response to a possible enemy attack," Oleksandr Syrskyi said without elaborating, in remarks released by his office for publication on Sunday.

The Ukrainian military said the activity in this area prevented Russia from sending a significant number of its forces to Ukraine's eastern region of Donetsk, where some of the heaviest fighting has taken place in the more than three-year-old full-scale invasion.

Syrskyi's troops are repelling Russian forces along the frontline, which stretches for about 1,200 km, where the situation remains difficult, the Ukrainian military said.

Russian gains have accelerated in May and June, though the Ukrainian military says it comes at a cost of high Russian casualties in small assault-group attacks.

While the military says its troops repelled Russian approaches toward Ukraine's Dnipropetrovsk region borders last week, the pressure continues in the country's eastern and northern regions.

The Russian military also continues its deadly drone and missile attacks on the Ukrainian cities further from the front, prompting Ukraine to innovate its approaches to air defence.

Ukraine's military said it currently destroys around 82% of Shahed-type drones launched by Russia but requires more surface-to-air missile systems to defend critical infrastructure and cities.

The military said the air force was also working on developing the use of light aircraft and drone interceptors in repelling Russian assaults which can involve hundreds of drones.

Ukraine also relies on its long-range capabilities to deal damage to economic and military targets on Russian territory, increasing the cost of war to Moscow.

Between January and May, Ukraine dealt over $1.3 billion in direct losses in the Russian oil refining and fuel production industry, energy and transport supplies as well as strategic communications, the Ukrainian military said.

It also dealt at least $9.5 billion more of indirect damages through the destabilization of the oil refining industry, disruption of logistics and forced shutdown of enterprises, it added.

It was not clear whether the Ukrainian military included the damages from its operation "Spider's Web" which damaged Russian warplanes -- and Ukraine said cost billions in losses -- in the estimates.

(Reporting by Sergiy Karazy; writing by Anastasiia Malenko; Editing by Bernadette Baum)