KYIV, Ukraine (AP) — Russian authorities struggled 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 meters, according to Russian state news agencies. There are 500 firefighters involved in the operation, and 41 of them already have been hospitalized with injuries, according to the Russian state-owned news agency TASS, citing local officials.

Ukraine’s Army General Staff claimed responsibility 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 Zelenskyy 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," Zelenskyy told diplomats 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.”

Zelenskyy said the Ukrainian army has captured 1,250 square kilometers (480 square miles) 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,” Zelenskyy said. “Our proactive defense 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 on the city of Pokrovsk and forcing Ukrainian forces to pull back and Ukrainian civilians to flee their homes.

Russia’s relentless six-month slog across Ukraine’s Donetsk region following the capture of Avdiivka has cost Ukraine heavily in troops and armor. Ukrainian defenders have no choice but to pull back from positions blown to pieces by Russian artillery, missiles and bombs.

Pokrovsk is one of Ukraine’s main defensive strongholds and a key logistics hub in the Donetsk region. Its capture would compromise Ukraine’s defensive abilities and supply routes and would bring Russia closer to its stated aim of capturing the entire Donetsk region.

Russia wants control of all parts of Donetsk and neighboring Luhansk, which together make up the Donbas industrial region.