The Ukrainian military said it had captured a key transit point for gas supplies to Europe, marking Kyiv’s most ambitious incursion into Russian territory in its decade-long war and forcing Moscow to impose a state of emergency in two regions.

The unexpected offensive, which entered its fourth day on Friday, is the largest attack by Kyiv’s forces on Russian soil not only since President Vladimir Putin began his full-scale invasion of Ukraine in 2022, but at least since the Kremlin’s covert invasion of Crimea and Donbass a decade ago.

The attack is aimed at diverting Russian troops from the east, exposing the country’s weaknesses and strengthening Kiev’s position in future negotiations with Moscow, a government adviser said, after months of Russia’s successes on the more than 1,000-kilometer-long front of the grueling war in Ukraine.

Late Friday, in the first public confirmation of the incursion into Ukraine, a military brigade released a video of its soldiers in the office of Russian state energy company Gazprom at the Sudzha gas metering station, a key transit point for gas supplies to Europe.

“Sudzha is under the full control of Ukrainian troops,” the 61st Mechanized Brigade said on Telegram. Gas prices in Russia have risen sharply to an annual high, a Gazprom representative said on Telegram.

A state of emergency was declared in the Russian regions of Kursk and Lipetsk, and Ukrainian forces were involved in heavy fighting there.

A drone strike on Friday added a complicated new dimension to the incursion, dwarfing several previous cross-border attacks by anti-Moscow Russian volunteer fighters and a far-right militia under the command of Ukrainian military intelligence.

Some military analysts questioned the timing of the Kursk operation and the deployment of some elite units, as the Ukrainian army is already struggling to defend the front line in the Donetsk region.

Elements of at least four Ukrainian tank and airborne brigades have taken part in the operation so far. Videos verified by the Financial Times and military analysts show them using US Stryker and German Marder fighting vehicles provided to Kyiv as part of multibillion-dollar military aid packages.

American and German officials said the deployment of the armored vehicles in Russia did not violate their terms of service, although Washington and other Western governments had previously objected to the use of such weapons in Russia, fearing an escalation of the war by Moscow.

As Kyiv continued its incursion, Russia responded on Friday with an attack on a busy supermarket and post office in the eastern Ukrainian town of Kostiantynivka, killing at least 12 civilians and wounding 44 others, President Volodymyr Zelensky and local authorities said.

Officials released videos showing black smoke rising from a destroyed store and rescue workers rescuing customers trapped under rubble. Another video showed seriously injured people sprawled on the sidewalk.

The nighttime drone attack on Russia was carried out early Friday by the Ukrainian security service SBU together with the military and special forces, a Ukrainian official with knowledge of the operations in Russia told the FT.

The official said the aim of the attack on the Lipetsk air base – about 300 kilometers from the international border and just east of the recent fighting – was to “destroy Russian air logistics so that the enemy has no opportunity to bomb Ukrainian cities with anti-aircraft missiles.”

Several warehouses filled with munitions were blown up, the official said. Videos posted on social media and geolocated by the FT showed massive explosions reaching into the night sky.

The Ukrainian official claimed that up to 700 glide bombs stored in the warehouses were damaged or destroyed. Several dozen fighter jets, including Su-34, Su-35 and MiG-31, as well as military helicopters were also at the air base, the Ukrainian Army General Staff said.

“Most of the planes stationed at the military airfield … did not have time to take off,” the Ukrainian official claimed.

The FT could not immediately confirm whether the bombs and aircraft were damaged or destroyed. Russian military bloggers reported that no aircraft were damaged.

Videos shared on Russian Telegram channels showed kilometer-long columns of civilian vehicles fleeing east from the Lipetsk and Kursk regions.

The Ukrainian official said the attack on Lipetsk was a follow-up to an attack on Monday on the Morozovsk military base in Russia’s Rostov region, which destroyed anti-aircraft missiles and jet fighters.

The Ukrainian General Staff said its forces had also attacked Russian anti-aircraft missile divisions in the occupied territory in eastern Donetsk.

These attacks came as Ukrainian forces continued their assault on the neighboring Kursk region, where the Kremlin has lost control of an area of ​​about 350 square kilometers, according to calculations by the FT and military analysts.

Alexei Smirnov, the acting governor of the Kursk region, said the situation remained “difficult.” He said his government had declared a state of emergency and was still in the process of evacuating residents.

Pentagon deputy press secretary Sabrina Singh told reporters in Washington on Thursday that Ukraine was taking “measures to protect itself” and that the Biden administration did not view the incursion as escalating.

Video and photo evidence suggests that the Ukrainian army has advanced as far as 35km into Russia from the international border. A video circulating on social media, located by the FT on a highway in Rylsk, shows a destroyed column of Russian military vehicles carrying soldiers stretching hundreds of metres long. The bodies of several soldiers can be seen.

A person familiar with the operation provided the FT with a video that allegedly shows a drone equipped with a first-person view (FPV) camera and armed with explosives crashing into the tail rotor of a Russian military helicopter.

The person said the SBU was behind the attack – the second Ukrainian FPV drone attack on a Russian helicopter this week. The person said both helicopters crashed, but the FT could not independently confirm the claims.

On Friday afternoon, Russian state media showed footage of large convoys of military trucks transporting heavy weapons to the fighting in Kursk.

Zelensky did not comment explicitly on the incursion, but thanked Ukrainian troops on Friday for “destroying the Russian occupiers, holding the front line and ensuring that Ukraine remains on the world map.”

“We are doing our best to give our warriors as many opportunities as possible to end this war as quickly as possible with a just and lasting peace,” he said.

Andriy Zagorodnyuk, a former Ukrainian defense minister who advises the government, told the FT that Kyiv had planned the operation well in advance. Zagorodnyuk said its aims were to distract Russian troops fighting elsewhere in Ukraine, as well as to bring the war closer to the Russians and deter them from supporting the war effort.

The aim was also to expose Russia’s weaknesses, including its inability to protect its own border, and to try to seize the initiative a year after an unsuccessful counteroffensive and months of Russian successes.

⁠Zagorodnyuk said the Ukrainian military had demonstrated its ability to conduct “new tactics of combined arms operations” taught to it by Western military instructors.

He said the goal was not to capture Russian territory and hold it “for a long time.” “We don’t need Russian land,” he said. “We want them to fail on ours.”

Konrad Muzyka, a military analyst at Rochan Consulting, a Poland-based security group, said the Ukrainian operation could strengthen its position in the war if it forces Russia to withdraw resources from eastern Donetsk and allows Kyiv to maintain its presence in Russia’s Kursk region.

This presence could provide a better negotiating position in the future, he said.

“However, if the Ukrainian troops are pushed back from Russian territory without tangible results and with heavy losses and the Russians continue to advance towards Pokrovsk (in Donetsk),” he said, then the top Ukrainian military leadership would have taken a great risk.

“There is no middle ground here. The operation is risky,” he said.

Ukraine also said on Friday that it had landed on the Kinburn Spit, a strip of land jutting out into the Black Sea that has been occupied by Russia for two years.

A video posted by Ukrainian military intelligence showed troops landing on jet skis. “The Kinburn Spit will be free, like all other temporarily occupied territories of Ukraine,” an official post on Telegram said.

Additional reporting by Max Seddon in Riga, Anastasia Stognei in Tbilisi and Isobel Koshiw in Kyiv