KYIV, Ukraine — 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 Kyiv’s forces also pushed into Russia’s Kursk region.

The fire at the depot in the town of Proletarsk burned across an area of a hectare (2 1/2 acres), 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.”

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Ukrainian Forces Advance in Russia’s Kursk Region

Hundreds of kilometers (miles) to the northwest, Ukraine continued its operations in Russia’s Kursk region.

Ukrainian President Volodymyr Zelenskyy said the daring Kursk incursion has allowed his army to capture a significant number of Russian prisoners who could be used in exchange for captured Ukrainians, expanding on Kyiv’s objectives for the mission launched two weeks ago. He earlier explained that Ukraine sought to create a buffer zone that might prevent further cross-border attacks by Moscow, 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.

The Institute for the Study of War, a Washington-based think tank, reported that Ukrainian forces “continued to marginally advance” in Kursk on Monday.

Ukraine’s incursion, the largest attack on Russia since World War II, has exposed Russian vulnerabilities under the leadership of President Vladimir Putin.

“Our defensive actions across the border, as well as 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.”

TASS reported that 17 people have died and 140 have been injured in Ukraine’s incursion, citing an unnamed source in the Russian medical service. Of 75 people hospitalized, four are children.

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Strategic Bridges Destroyed in Kursk Region

Ukrainian and Russian reports suggest Ukraine has destroyed or damaged at least three bridges over the Seym River in the Kursk region — a move which could potentially trap Russian forces between the river, the Ukrainian advance and the Ukrainian border.

Over the weekend, Ukraine’s Air Force commander posted two videos of bridges over the Seym being hit, and satellite photos by Planet Labs PBC analyzed Tuesday by The Associated Press confirmed that a bridge in the town of Glushkovo had clearly been destroyed. There was visual evidence via satellite of mechanized maneuvers by forces in the Kursk region but no clear picture of the fate of the two remaining bridges.

A Russian military investigator confirmed Monday that Ukraine had “totally destroyed” one bridge and damaged two others in the area.

“As a result of targeted shelling with the use of rocket and artillery weapons against residential buildings and civilian infrastructure in the Karyzh village … a third bridge over the Seym River was damaged,” an unnamed representative for Russia’s Investigative Committee said in a video published on the Telegram channel of Russian state TV anchor Vladimir Solovyov.

Pontoon bridges — temporary bridges used by militaries when formal bridges are blown out — could be seen in the satellite images in two different positions along the Seym River in recent days. The pontoons likely were built by Russian troops trying to supply forces around the Ukrainian advance.

One pontoon bridge appeared along the serpentine path of the river between Glushkovo and the village of Zvannoye on Saturday, but not in images taken Monday. On Monday, smoke could be seen rising along the banks of the river nearby — typically the sign of a strike.

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Challenges in Eastern Ukraine’s Donbas Region

But even as Zelenskyy hails successes in Kursk, his troops face a bleak situation in the Donbas region in eastern Ukraine, where Russia is bearing down on the city of Pokrovsk and forcing Kyiv’s 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 both sides heavily in troops and armor.

Russian forces are heading towards Pokrovsk, 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.