Russian authorities struggled on Tuesday to put out a massive fire in the southern Rostov region for a third consecutive day after an oil depot was hit by Ukrainian drones as Ukrainian forces push into Russia’s Kursk region.

The fire at the depot in the town of Proletarsk burned across an area of 10,000 square metres, according to Russian state news agencies. There are 500 firefighters involved in the operation, and 41 of them already have been hospitalised with injuries, according to the Russian state-owned news agency TASS, citing local officials.

Ukraine’s Army General Staff claimed responsibility on Sunday for attacking the oil depot, which was used to supply the needs of Russia’s army, calling it a measure “to undermine the military and economic potential of the Russian Federation”.

Ukrainian President Volodymyr Zelensky said the daring Kursk incursion has allowed his army to capture a significant number of prisoners who could be used in exchange for captured Ukrainians, expanding on Kyiv’s objectives for the mission launched two weeks ago. He earlier said that Ukraine sought to create a buffer zone that might prevent further attacks by Moscow across the border, especially with long-range artillery, missiles and glide bombs.

“Overall, this (Kursk) operation became our largest investment in the process of freeing Ukrainian men and women from Russian captivity,” Zelensky told diplomats on Monday, according to a statement published on Telegram late in the day. “We have already captured the largest number of Russian prisoners in one operation.”

Zelensky said the Ukrainian army has captured 1,250 square kilometres and 92 settlements of Russia’s Kursk region. Ukraine’s incursion into Kursk, the largest attack on Russia since World War II, has exposed Russian vulnerabilities.

“Our defensive actions across the border, as well as (Russian President Vladimir) Putin’s inability to defend his territory, are telling,” Zelensky said. “Our proactive defence is the most effective counter to Russian terror, causing significant difficulties for the aggressor.”

But as he hailed successes in Kursk, his troops face a bleak situation in the Donbas region, where Russia is bearing down Pokrovsk and forcing Ukrainian forces to pull back and Ukrainian civilians to flee their homes.