KYIV, Ukraine (AP) — Ukrainian long-range drones struck one of Russia’s biggest oil refineries for the second time in three days, a senior Kyiv official said Monday, as Ukraine tries to slow the Russian army’s push along parts of the front line and the third anniversary of the war approaches.

The attack late Sunday hit a refinery in the Volgograd region, which is one of Russia’s 10 biggest refining facilities, processing close to 6% of the country’s oil, an official in Ukraine’s Security Service told The Associated Press.

Russian authorities acknowledged only a brief fire at the Volgograd refinery during the drone attack.

Ukrainian defenses are creaking under a monthslong Russian drive to occupy more land, especially in Ukraine’s eastern industrial heartland, before the possible start of peace negotiations steered by U.S. President Donald Trump.

Ukraine, though heavily dependent on Western military aid, has been developing its own arms industry, including drones that can fly increasingly long ranges with bigger payloads.

Ukrainians authorities said they attacked the same Volgograd refinery on Friday, but no further details were immediately available.

The Sunday night attack by Ukrainian drones also targeted a major natural gas processing plant in Russia’s Astrakhan region, the security service official said on condition of anonymity to discuss classified operations.

The official said that the attack was the security service’s fifth successful operation behind the front lines this year as it looks to disrupt the Russian war machine.

The Volgograd refinery is located around 480 kilometers (300 miles) behind the front line. The Astrakhan plant is even further away, roughly 800 kilometers (500 miles) from Ukraine.

The Russian military said that it destroyed 70 Ukrainian drones over six Russian regions during the night.

Russia’s Defense Ministry said that the Volgograd region and the Astrakhan region on the Caspian Sea coast were targeted, as were sites in the Belgorod, Kursk and Voronezh regions on the border with Ukraine.

Authorities temporarily suspended flights in and out of nine Russian airports in different regions.

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